///// Additional aero mods and findings /////

[Andyinchville1]

Hi All,

Got excited to go to North Carolina to visit family this weekend and decided to take advantage of such a long trip to do a couple aero mods that I've been putting off for a long time.

First arrow mod was a cardboard belly pan to fill the area in between the black plastic of the underside of the front bumper making a smooth transition to the aluminum skid plate...

Normally the skid plate has a step in it which I often thought would cause bad aerodynamics so putting the cardboard between the black plastic underside of the bumper straight to the transition in the belly pan makes for a smoother airflow in my opinion (the cardboard extending from the front to the lower part of the aluminum pan eliminating the "step" facing the air and causing undue turbulence).

Additionally there's a couple small open sections on the sides that I wanted to cover up also .. there's a open section beneath the intercooler which looks like it could be a turbulent area and there's an equivalent side on the other side of the car except that there is no intercooler on the driver side.

I also ordered an extra set of gas pods and increased the number of gas pods that I had running across the car on the rear .... Gas pods are basically vortex generators.

Anyways on a 413 mile trip to and from North Carolina on approximately an 88° day I managed 55.6 miles per gallon driving at 62 mph with the air conditioning on all the way down and on approximately 20% maybe on the way back (maybe it wasn't 20% I didn't use very much on the way back maybe even 10%?).

In any event I'm thinking the MPGs with any air conditioning use was still pretty impressive.

One thing I noticed on the way down was that my water temperature according to the scan gauge would be 191° to 193° with the addition of the cardboard belly pan whereas before the belly pan mod was done I would typically run a rocksteady 190° at cruise.

Additionally even though I run a vented fender liner covering up the area underneath the intercooler cause my air intake temperatures to go up a fair amount

Whereas normally I would be approximately 30° over ambient when not pushing too hard ... the same with the cardboard blocking the underside of the intercooler ended up raising the air intake temperature approximately 45 to 50° over ambient during the day

maybe the area underneath the intercooler needs to be left open even though I have a vented fender liner for lower air intake temps?

Just curious and I've read that obviously it's better to have cooler intake temperatures than not ....

Do you think the extra 15 or 20° of air intake temperature affects economy in a negative sense? If so how much loss in power or efficiency for approx a 15° to 20° increase in air intake temperatures?

Of course I could try to do another run leaving the cardboard block off underneath the intercooler but then again lately I haven't been able to do a lot of long distance driving unfortunately most of my work recently has been more van work rather than car work.

In any event I believe these aero changes have had a positive effect ....

I'm going to see if I can find a sheet of ABS plastic and take the cardboard off and make a more permanent belly pan for the front of the car ...

I thought about using coroplast and may end up going with that but I'm thinking that abs plastic may be a more permanent solution.

Anyways based on this one test (I know I need to do more but at least I filled up before the trip at the same pump I filled up with at the end of the trip so hopefully that would minimize variations in Phillips based on angle of parking, pump etc ) I would say that drag
Was reduced and MPGs went up

I'll try to post pictures here but I'll post this first because I don't want to lose all this dictating.

Note: I did fill in the little corners also with cardboard but I don't think this picture reflected that since it was a picture while everything was in progress

I think theoretically the gas pods should go further back on the tail light itself but the gas pods are magnetic so they won't stick there without gluing them on and the police may give a problem with blocking tail lights so I took the easy way out and simply let them stick a little further back then optimal well a little further forward I should say then optimal

 

[Windex]

Those gaspods look familiar...

 

[hskrdu]

Andy, I love your experimentation and attempts to document your pursuits. As I said in your other thread, some of the notable differences between the E-Golf, Bluemotion, and other variants and their Euro cousins is the amount of underbody aero coverings. There is no doubt these can contribute to improved FE under the right circumstances. At the same time, when I read your process and evaluation, it's more anecdote than science. When you say (after a single run) that your results indicate an improved Cd or FE, it steers away from objectivity and towards placebo effect. We know that real-world testing, for most private owners, makes it nearly impossible to conduct reliable (repeatable) evaluations of the impact of variables on FE, so I just suggest restricting "I think this worked" to an "opinion" section, and focus on gathering and reporting data. Here's some thoughts from reading your post:

Minimize the variations by filling up at the same Phillips station? When you talk about specific pump and parking angles, it makes me think you are not filling to the point where you let the fuel settle and see it settle at the top. If you aren't doing this, every time, in the same manner, then you don't know how much fuel you used, and your FE calculations will be incorrect. Without the slow and exact process of filling up to the same point, every single time, your numbers will always be in doubt.

Why are there gas pods on the side? I know I've mentioned the years of aero testing other owners have done, and I think you've read some of that, and all the wind tunnel and field testing I've seen points to Cd being lowered through lowering drag at the upper rear of the roof. Have you seen studies that show different results? Just curious, I don't keep up on this stuff.

I've mentioned the VAG OEM underbody trays that GolfDave and others have added to their Mk7's, which are directly from the E-Golf or Bluemotion / high-FE Euro variants. The specific versions VW makes for Europe indicate that their testing has focused on areas of the underbody behind the first tray, with numerous variations of partial (half) under-engine trays with openings for cooling, etc. VW also focuses on the nose, with the E-Golf and BM variations getting grills that reduce Cd. My guess is that your visual evaluation of where to put the cardboard is focused on the wrong areas (and that cardboard isn't the right choice, as you said).

The best way to get valid results associated with FE testing is to evaluate one variable at a time, and one way to have results that are valid and reliable is to eliminate all other variables. The former can be controlled, the latter is nearly impossible in "real-world" testing. Not only doesn't a single run of 413 miles constitute testing of a single tank (a minimum baseline that we often use to discuss an owner's FE concerns), but it ignores the basics of producing valid and reliable results. Changes in ambient temperature will raise or lower your FE, from impact on engine, Cd, or on fuel characteristics. Wind direction and speed can elevate or crush FE. Your right foot, and how it is applied on flats, hills, and stoplights can create dramatic swings in FE. Weather, humidity, dew point, and storms can swing FE numbers. Gear selection, AC use, use of other car equipment all impact FE. If you don't have precise numbers (minutes of AC use over total engine running minutes) you don't have reliable data. Amount, type, and proximity of traffic near you can create notable swings in FE. I'll leave aside things that you have controlled, such as route (to NC and back on exact same route), or other changes to your car (tire height, width, raising or lowering suspension, etc.) where you have established recent consistency. "Real-world" testing (testing that isn't particularly well-suited to FE science) includes variables that are nearly impossible to control, producing results that are generally not valid or reliable, until an owner (or owners) have accumulated lots and lots of miles. A final example: Your tires, with each mile, are wearing away, reducing their circumference, and at the same time reducing their rolling resistance. The smaller diameter tire indicates greater distance traveled, raising indicated FE, contributing to both to inaccurate FE observations, and attribution of increased FE to the wrong variable. Additionally, the lowering rolling resistance increases FE, which (while not contributing to inaccurate FE) also leads owners to tie increased FE to the wrong variable.

As said, I love your "experiments" even if they are not quite scientific. I'd just rather see more data and documented process (numbers, gallons, procedures, data) which indicates that your efforts are in any way connected to your results. Otherwise it's all just flames painted on the side, and a dose of Fuel Boost (TM) in the tank (a search will reveal more).

Lastly, all FE reporting is done within the perspective of that particular car's baseline, so if you aren't publishing baseline (re-established after each of your mods (tires, suspension, wheels, tune, etc.), then the reader has no perspective for comparison, and you just get members telling you that everything is "snake oil" ( makes you think they don't understand the term). My MkIV's best single tank FE was between Texas and Maryland, averaging 67MPH over 800.6 miles on 14.314 gallons (filled brim to brim) for 55.931 MPG. Would have been higher but we got stuck behind an accident for 45 minutes, creeping the whole time. Plus the car was packed full of heavy stuff. That was with zero mods, car completely stock, and a baseline established over the previous 25,000 miles. My point? Your mods might be having an impact, but we'll never really know unless you follow best practices and focus on data.

 

[Andyinchville1]

Those gaspods look familiar...

HA yes, the gas pods I got from you had babies ;-)

(Actually in coast down testing the ones I got from you they seemed to help (as measured by top speed coasting down a known hill with prior runs done without the pods) .... so in the spirit of "bigger is better" and "the more the merrier" I ordered another set and installed them .....
I believe they do help .... never made 80+ coasting down the hill before (of course so far only 2 runs with them down the hill BUT both of them have been faster than without).

Andrew

 

[Windex]

Andy, do take the time to read through Hskrdu's post. There is a lot of good information there.

As he points out. You want to minimize all variables as best as possible, and gain many pre and post mod samples in order to make your findings as clean as possible.

Always deal with completely full tanks, and miles driven. Pen and paper only. Have you done a ventectomy yet? It really is the only way to fill consistently every time.

Take a look at my fuelly. Chart all fuel up's. You will see much variability in my complete tank full economy #s even though I always fill to the brim the same way, and don't vary my driving style much.

 

[TDeanI]

You do need to get the heat out of the engine compartment. The increase in air intake temperature will help gas mileage but the added heat in the engine department is not good for your tranny. Leave an opening (or make one) in your bottom engine cover to allow air that already went through the radiator to get out. The best place is under the car by the undertray where the air velocities are high and the air pressure is the lowest. Then you get the added flow through the radiator as air moves from the high pressure area (front) to the low pressure area.

I have a full undertray (except around the cat converter and the tunnel around it). I have enough openings in my bottom engine cover for cooling air to get out and exit through this tunnel around the cat.

Removing your roof racks and passenger mirror will get you a few more mpg by reducing frontal area and reducing coefficient of drag. Another cheap mpg mod is to add some small deflectors on the edge of the hood to get air flow to jump over your wipers blades.

Too many vortex generators will increase drag by increasing frontal area. Vortex generators work best when you are trying to get turbulent flow to attach back to the body, so use vortex generators:
by side mirrors,
before a wheel opening,
or on the roof line of a sedan to get flow back down onto a deck spoiler for down force
Under the car before suspension components if you have an undertray.

They do work better than nothing, but there are better ways to reduce drag.

You would be better to install a nice body line edge or lip along the rear hatch side and top body lines for the flow to detach cleanly from the body. Look at a Prius, and you will see what I am talking about. Even the taillights and bumper have a nice edge for flow detachment. All new wagons and SUVs have a top wing edge that gives that nice crisp detachment point.

The rounded edges of your hatch and bumper allows a strong vortex to form as flow curves around them and is causing low pressure (suction that is trying to hold your car back). The crisp separation edge will reduce the vortices and drag. Best way to learn aerodynamic tricks is to study what the leaders in low coefficient of drag are doing, Tesla, Prius, Hyundai hybrids, Corolla and Camry Hybrids, Volt, Leaf, Mercedes, etc. and see what they are doing.

The new cars with the best mpg (or electric) are using curtains, diffusers, full body pans, trailing edges, flat wheel covers, retractable wipers, etc.

 

[Andyinchville1]

Hi

Thanks for all the input and follow-up so far .... I'll try to answer or address a few items here.

1) As far as the vortex generators across the top of the car , yes I do have those going across the top as well ....

The instructions for the gas pods called for a 4-in spacing on the pods so I ended up after thinking about it having a total of three kits so far on the car ...

Across the top initially and then part way down the sides and then more down the sides after the third set.

Theoretically, I could use one extra set so I can go further down on the bumper.

It was a little hard for me to get the extra sets because they were kind of expensive .... you only get nine of them for about 70 bucks if you can find them cheap online.

I think the air tabs are quite a bit cheaper but I didn't start off with them or have any experience with them.

2) as far as sharp edges and other aero mods to promote cleaner separation of air etc I had been lurking around the eco-moder site for a while and I thought about building a box cavity on the back extending out maybe two or so feet .... I think that would go a long way towards helping aero but it's also quite a bit of fabbing to make it look nice.

On the other hand maybe I'll just crudely Fab something up using some 1x2 wood , hooks and straps , cardboard etc to get it on to the rear hatch and at least try to see if it helps first before making it look nice.

3) I haven't seen if there's any replies on my other thread yet but I am trying to take the mirror heads off the car and simply running the small spot mirrors and a very tiny 1x2-in regular mirror so as to stay legal (I was told in Virginia that convex mirrors alone are not allowed ...you have to have "regular" mirrors also (non distorted non "fish eye" that you get when you're running small convex mirrors).

This mod should help with arrow Dynamics by reducing the amount of square inches facing forward.

Actually the other idea I had was to put some spotlighting or mounting led lights where the mirror heads used to be ....
I have some small driving lights by rigid industry lights ....

Yes putting the small lights on there would add more square inches of frontal area but I drive a lot at night and I think the extra lighting power could come in handy on the other hand I could just simply Mount them on the front of the car but I think mounting lights wide and on the side would help illuminate the sides of the road better where deer like to hide .... Ultimately though I believe I probably will go with just mirrors for the smaller frontal area.

4) I had read that roof racks are bad for aero drag when you have the cross members on but since I don't have the cross members on I'm guessing removing what's left should yield some small amount also. ...

Since my headliner is sagging slightly I'll wait until I take it to the upholstery shop for them to remove the headliner and then I'll delete the roof racks actually I should probably do that fairly soon before it gets too cold

4) thinking about removing the black plastic bottom of the rear bumper

When I looked under the bottom of the car it seems like that extra plastic hanging down could be some additional resistance as it forms a slight pocket underneath

I had watched some videos on YouTube where some of the tuner guys actually cut away half of the rear bumper to try to eliminate that

I'm not sure I'll go quite that far but my plan is to maybe take off that black piece and get some extra ABS plastic sheeting and making a smooth underside along with some sort of diffuser

As a side note I did price a 4x8 sheet of ABS plastic ... black color textured on one side , smooth on the other.... 3/16 of an inch thick for $135 a sheet

An interesting fact is the same company would sell me a quarter inch sheet of that for over $800 a sheet so it's a huge price difference for an extra 16th of an inch thickness.

On the flip side of 4x8 sheet of coroplast is only $30 at the sign shop but I'm guessing the ABS plastic will be far more durable overall than the chloroplast for some underbody panels.

5) as far as being more scientific on my testing , I agree it would be nice to be able to compare apples to apples when doing testing and only making one change at a time when doing mods but sometimes that's hard to do with time constraints etc.

When I do go on drive so it's mainly for work and I try to drive a steady 62 mph so that at least helps to even out what speed I'm operating at even though the terrain may be different

I guess I arbitrarily chose 62 because a lot of roads are 55 and doing 62 won't generally get you a ticket and I guess a lot of roads that are 70 miles an hour driving 62 although slow is not excessively slow I guess

I'm trying to keep notes on how much air conditioning is being used because I believe (again using gut feel) that I lose probably two to three miles per gallon sometimes using the AC that's just the subjective thing in my head...

I'm not driving as many long-distance trips as I used to so it might be hard to actually come up with a number on that.

6) as far as pumping gas I do use the same gas station in the same pump parking in the same direction to help minimize potential variations in fill.

I am vented as far as tank but I have not been patient enough to literally fill the fuel to the brim
But rather I pump it until it clicks wait approximately 10 seconds pump it again until it clicks and then do one more pump until it clicks again

I tried to keep that pattern of feeling to at least have some form of consistency in filling.

The other reason I haven't typically filled to the brim is because I typically fill my tank at the end of my run

As an on demand Rush courier is typically best to have the car ready to roll at a moment's notice rather than having to drive to a gas station to fill up before a run because the companies on some of the stat runs are pretty tight with time

The other thought I had if I filled to the brim was possible fuel expansion in the tank if I filled it to the very brim and let it sit through the heat of the day maybe the expansion would cause it to overflow or something.

7) as far as keeping records, I do have a fuelly account and approximately 30,000 miles worth of data and fill ups ...
I think if I remember correctly My overall mileage was approximately 53 MPGs and maybe there was one or two 56 and 57 mile per gallon tanks in there.

Admittedly I've been somewhat negligent in keeping an updating my fuelly lately only because I've been driving my van a lot for work instead of the car.

I guess also so it's not too contaminate my fuel information I haven't been putting in a lot of the tanks that I've driven in the car intentionally because I just been puttering around town a lot in the small car whereas most of my fuel emailage was when I was doing long-distance drives or longer distance drives for the company so I think if I put in my town driving and local driving that would kind of corrupt the fueling numbers so I'm trying to do tanks that are mainly highway runs on the fuelly ....

8) one thing I would like to do to test anything aerodynamic is to run the car is to run the hill several times in a row ...

Of course doing that I think I would have to make a dedicated time to do that because normally I'm just on the hill as part of a drive and not necessarily trying to get several data runs on the same trip.

Today I was on the mountain three different times in the same day but I was not able to get really good accurate data because of traffic.

To help with gathering more data I did jot down speeds at various points coasting down the hill where I wasn't going as fast because I think that would be more obtainable rather than overall terminal velocity at the bottom of the hill because terminal velocity means going a fair amount faster than regular traffic which you really can't do in traffic but you could probably maintain or hit 70 and follow along with traffic fairly easily.

Of course I'm thinking about that I did try to maintain distance between other traffic because I didn't want to contaminate the speed reading because of drafting

Yes to do things 100% right as far as obtaining data etc is a pretty difficult endeavor and most of my data gathering comes just from my normal driving

On the flip side I will say this my last two tanks have had really good gas mileage...

I left them in the car but I think one of them was 55.6 MPGs and today's was 57.6 which I think was almost an all-time high but I'll have to look through all that and I'm not sure if it was 57.6 or not but it was in the 57 range

Hopefully I'll get to do a few more runs long distance and gather some more data...

It did rain today so my cardboard underbelly has taken some damage I think what I'm going to do is take it off so I have my pattern before it totally falls apart or gets damage to the point where I can't make a tracing of it onto either chloroplast or ABS plastic I think I'm going with the ABS plastic bow because I think that's a more permanent solution

Wow I didn't know this was getting so long....

Looking forward to trying to do more aerodynamics wise I'll keep you guys posted and thanks for the input

Andrew

 

[johnsTDI]

my 13 TDI Jetta gets 2 - 2.5 Liters per 100km's of driving better fuel economy on the highway better fuel mileage by simply increasing the Cetane rating a little higher i added a bottle of Kleen flow Cetane boost. my TDI's compression is at 16.1 so a bump in cetane the engine loves it.

 

[hskrdu]

1) As far as the vortex generators across the top of the car , yes I do have those going across the top as well ....

I was suggesting that pods placed in the wrong areas might be creating drag. From what I've seen, more is not necessarily better.

2) as far as sharp edges and other aero mods to promote cleaner separation of air etc I had been lurking around the eco-moder site for a while and I thought about building a box cavity on the back extending out maybe two or so feet .... I think that would go a long way towards helping aero but it's also quite a bit of fabbing to make it look nice.

If only you had a wagon...
The aero extensions to the rear make more sense for a sedan, but the JW has one built-in. One reason the ALH JW was originally stickerd by the EPA as having higher FE than the Jetta and Golf was lower Cd. I'm guessing taping the seams and smaller svm's would have a greater impact.

3) I haven't seen if there's any replies on my other thread yet but I am trying to take the mirror heads off the car and simply running the small spot mirrors and a very tiny 1x2-in regular mirror so as to stay legal (I was told in Virginia that convex mirrors alone are not allowed ...you have to have "regular" mirrors also (non distorted non "fish eye" that you get when you're running small convex mirrors). Actually the other idea I had was to put some spotlighting or mounting led lights where the mirror heads used to be ....
I have some small driving lights by rigid industry lights ....

Not counting VA law, eliminating or reducing the size of the svm's will certainly help. Taking the caps off will likely just increase drag and negate smaller mirrors. Adding lights which increase drag will not help. I would suggest the stubby euro mirror mod which will reduce the forward surface area, keep the aero cover, and look better. Mixing the goal of reducing drag and also adding lights for visibility seems to go back to my first post. Additional power draw from the engine will also lower FE.

4) I had read that roof racks are bad for aero drag when you have the cross members on but since I don't have the cross members on I'm guessing removing what's left should yield some small amount also. ...

Since my headliner is sagging slightly I'll wait until I take it to the upholstery shop for them to remove the headliner and then I'll delete the roof racks actually I should probably do that fairly soon before it gets too cold

Just have a plan to prevent water intrusion. The few JW's with roof rail deletes haven't especially noted higher FE, but I'm sure it can't hurt. Most of the drag from rails is in crosswinds, and highway driving creates wind resistance at the front, which is partially why crossbars and racks create a FE penalty.

4) thinking about removing the black plastic bottom of the rear bumper. When I looked under the bottom of the car it seems like that extra plastic hanging down could be some additional resistance as it forms a slight pocket underneath. I had watched some videos on YouTube where some of the tuner guys actually cut away half of the rear bumper to try to eliminate that.

That part is called the valance. My guess is that it improves overall aero, but as said above, a underbody tray that moves air smoothly along and away from the "pocket" area would certainly help. The valance (from memory) separates from the bumper by tabs, and unless I'm confused by your description, anyone cutting the bumper to remove the valance is hacking for no reason.

5) as far as being more scientific on my testing , I agree it would be nice to be able to compare apples to apples when doing testing and only making one change at a time when doing mods but sometimes that's hard to do with time constraints etc.

When I do go on drive so it's mainly for work and I try to drive a steady 62 mph so that at least helps to even out what speed I'm operating at even though the terrain may be different

I guess I arbitrarily chose 62 because a lot of roads are 55 and doing 62 won't generally get you a ticket and I guess a lot of roads that are 70 miles an hour driving 62 although slow is not excessively slow I guess

I'm trying to keep notes on how much air conditioning is being used because I believe (again using gut feel) that I lose probably two to three miles per gallon sometimes using the AC that's just the subjective thing in my head...

I'm not driving as many long-distance trips as I used to so it might be hard to actually come up with a number on that.

Sure, it's slow and painful to test one mod at a time. That was only one of my suggestions, and not the most important. Pick any speed that works, it's route and conditions which impact that speed. These are what need to be noted and tracked along with speed, as 62 on the flat with a tailwind is quite different from 62 in the hills with rain and wind. No need to guess on the AC impact, just log it with VCDS. Mine certainly sees a hit.

6) as far as pumping gas I do use the same gas station in the same pump parking in the same direction to help minimize potential variations in fill. I am vented as far as tank but I have not been patient enough to literally fill the fuel to the brim
But rather I pump it until it clicks wait approximately 10 seconds pump it again until it clicks and then do one more pump until it clicks again I tried to keep that pattern of feeling to at least have some form of consistency in filling. The other reason I haven't typically filled to the brim is because I typically fill my tank at the end of my run

As said above, consistency helps with long-term data, but not filling all the way to the neck or other methods will foster spikes in FE that look like they are due to aero mods, but are actually just from fuel measurements. It also produces one hit wonders (or several of them). My 61mpg tank was obviously not a 61 mpg road trip, but a day where a fueling variable was off.

The other thought I had if I filled to the brim was possible fuel expansion in the tank if I filled it to the very brim and let it sit through the heat of the day maybe the expansion would cause it to overflow or something.

Not a great concern with D2, which is why so many owners have done the ventectomy without any long-term problems.

7) as far as keeping records, I do have a fuelly account and approximately 30,000 miles worth of data and fill ups ...
I think if I remember correctly My overall mileage was approximately 53 MPGs and maybe there was one or two 56 and 57 mile per gallon tanks in there.

Admittedly I've been somewhat negligent in keeping an updating my fuelly lately only because I've been driving my van a lot for work instead of the car.

I guess also so it's not too contaminate my fuel information I haven't been putting in a lot of the tanks that I've driven in the car intentionally because I just been puttering around town a lot in the small car whereas most of my fuel emailage was when I was doing long-distance drives or longer distance drives for the company so I think if I put in my town driving and local driving that would kind of corrupt the fueling numbers so I'm trying to do tanks that are mainly highway runs on the fuelly ....

Not sure I quite understand, but IMO, all miles count. Roll every mile into your calculations and divide by total gallons of fuel used. I don;t have fuelly, I have "the Book,' which captures all costs, miles, fuel, and maintenance. (See my TCO thread).

8) one thing I would like to do to test anything aerodynamic is to run the car is to run the hill several times in a row ...

I didn't really follow the aero mod vs hill plan. Too many variables and too short a distance to really get accurate measurements, IMO.

Yes to do things 100% right as far as obtaining data etc is a pretty difficult endeavor and most of my data gathering comes just from my normal driving On the flip side I will say this my last two tanks have had really good gas mileage...

I left them in the car but I think one of them was 55.6 MPGs and today's was 57.6 which I think was almost an all-time high but I'll have to look through all that and I'm not sure if it was 57.6 or not but it was in the 57 range

I think you could probably guess what I'm gonna say....


Lastly (and I'm forced by the rules of Fred's to say this bc you are a veteran posting about fuel economy), you don't get gas mileage for your TDI, or pump gas at the fuel station. Keep us updated and drive safe!

 

[TDeanI]

I find the easiest way to see improvements (or lack of improvements) in aero mods is terminal velocity hill tests. Trip mpg works if you get enough of them to average together. There are many variables (wind, temperature, traffic, road slope).

I drive the same hills on the freeway every day to work, I start at the top of the hill and start the test at the same speed (60 mph), then write down my terminal velocity at the bottom at the same sign or the max velocity achieved, or I can go by the maximum distance coasted before I hit 50 mph. I can clearly see which mods let me gain increased velocity. I always use the same hill. Then recording this every day will give you an average and reliable figure to compare.

The problem with level road coast down testing or level road mpg readings for testing improvements is that roads are not 0.0% slope. Most roads that look level are 0.5% - 1% up or down when it seems like they are perfectly flat. Even telling a 1% slope from a 0% slope or a -1% slope is very hard. I have a Scan Gauge II and I can see if I am going up slightly or down slightly by looking at the instantaneous mpg. I can drive a road that I think is flat at 60 mph and look at my Scan Gauge II and get 45 mpg going one way and 75 mpg in the same spot going the other direction, so clearly there is enough slope to make calculating mpg inaccurate in one way testing.

I use that How Far Did I Run site to check road slopes. Click the car at the top and create a route on your testing road, then hit the profile button. You can see road slope changes in the profile by just moving your mouse. I found it is VERY hard around here to find any test area that is at least 1/2 mile long that has a constant slope or is perfectly level. Using this site, you should look at overall elevation gains or losses when comparing mileage for trips. A trip that loses 500' of total elevation will have better trip mpg than one with 500' overall elevation gain. So an accurate method of tracking trip mpg is go both ways, then average them together. Then do multiple runs to get accurate results. I would say at least 3 there and back. Five is better. I use my daily work commute for trip testing, both ways. Then average them all together. I use these results to compare my mods.

In my state, two rear view mirrors are required but one of those can be the rear view mirror and the other a side mirror. If you remove your passenger mirror you can supplement your view with one of the those oversize slightly curved rear view mirrors. They will let you see your right quarter pretty good. Or you can put a small mirror on the right side dash up against the window.