How much does weight affect fuel economy?

I think about airplanes, let's take a 747 for example with full fuel, it may cruise at first at maybe FL350 then as it burns more fuel, it gets lighter and may stair climb to FL390.

It got me thinking I think diesel weighs like 7.5 lbs per gallon so a fully loaded tank has somewher around[16gallons] 120lbs or so. Let's say you burn a good 8 gallons, you have now shed 57 lbs or so. The engine isn't pulling as much weight etc.

How much does this affect performance.
Let's model a 2000 GOLF 5SPD

Without getting complex
80MPH steady with a full tank 16 gallons, vs 80MPH with a half tank 8 gallons

Some but not a lot. Any adult passenger is going to weight 120 lbs (probably minium) and they will not reduce as you travel (lol)

Diesles tend to be unafected by load.

While mpg is very variable, the max load we ran in the Passat was 3 larger adults, back stacked to the roof with luggae, and the one passenger seat full of coats, purses etc. I would gues 800 lbs easy (and full fuel to start).

They got a trip meter read of 38 mpg on that trip (less than 3k on odometer, so early in break in).

Another trip up into our hilly North (lots of mountains, but the road is more hilly in the route it follows), we had 700lb load at least, and got 41 mpg.

Other factors have bigger influence (wind and travle speed)

The Gravity of the Situation

The powered aircraft has to fight gravity all the time. Weight is a disadvantage.
Non-powered aircraft (specifically soaring gliders) use gravity. With these weight is an advantage, once pulled aloft.
SoapBox Derby vehicles also use heavy weight to advantage, thus the maximum weight limit.
Your land use vehicle does not fight gravity all the time, just when climbing. Extra weight costs going up hill (and acceleration), aids in adding distance going down hills.
The net result is only a slight economy loss during steady speed driving as the result of increased tire flex and higher rolling resistance.

tdicruz33 said:

How much does this affect performance.
Let's model a 2000 GOLF 5SPD

Without getting complex
80MPH steady with a full tank 16 gallons, vs 80MPH with a half tank 8 gallons

Click to expand...

It won't affect much at steady-state.

force=mass*acceleration. Rearrange for acceleration:

a=f/m

If mass decreases from a full Jetta tank (17 gallons) to a nearly empty tank (2 gallons), net mass of the vehicle drops about 3.2%, therefore, 3.2% more force must be applied to maintain a given level of acceleration. {I included 175 lbs for a driver in this calc and used 3200 lbs for the car mass.}

So perhaps in heavy stop-n-go city driving fuel economy is decreased by 3% or so...but obviously there are many other factors, optimal engine temperature being the most important among them.

i think city driving weight would be more of a factor than cruising on the highway. i've always worried about reducing weight to improve economy (in my jetta i kept the spare tire/jack in the garage). however, after moving from MA to MN (1750 miles) in a fully loaded 05 jetta tdi, and i mean packed to the brim! - the rear of the car was sagging visibly, i'm not too concerned. average speed was probably around 70ish, some faster, and some slower, and about 6 hours of lots of construction zones, my avg for the trip was 44 mpg. i'm guessing the weight probably only took away 2-3 mpgs for those driving conditions...

Don't forget rolling resistance. I can't remember my friction physics, but I know this increases linearly with speed and mass.

(something like f=kmv, where f is friction)

Though I should note that wind resistance increases with the square of speed, and mass has no effect on this. Once wind resistance becomes the dominate factor (generally between 50 and 80 mph), you can ignore everything else. Until wind resistance takes over the faster you go the better, because engine losses are related to RPMs (and gears mean you can keep rpms low), and significant enough that you can get better milage by optimizing that factor (though not significantly).

So if you have a heavy car you can go faster before wind resistance overtakes the other factors. If your car is light your max speed before wind resistance takes over is slower.

The bottom line: your car will physically break long before mass has any significant effect on milage.

In airline operations, the tankering of fuel is used often due highly variable prices internationally. We have software programs in dispatch to calculate the savings...and they can be rather substantial. But several factors are different for a car. An airliner can usually carry about 1/3 of it's weight in fuel...so as a percentage of total weight ...this is significant ....whereas a car reduces its total weight as fuel is burned to a much smaller degree. An aircraft would cruise at a lower altitude initially then climb as fuel is burned...on an ocean crossing...you might climb 2 thousand feet every couple of hours...a car can't look for less dense air (less drag). For an aircraft such as an Airbus 310, a rough guide is 3% of the extra fuel weight per hour. eg 10,000 lbs of extra fuel results in burning 300lbs per hr more ...just to carry it.
My experience with the TDI is anything you can do to reduce weight is only marginally helpful....now put a roof rack/bike rack or trailer into the equation and you will certainly see your "Burn" go up.

Good question though!
Cheers
Ted

hank miller said:

Once wind resistance becomes the dominate factor (generally between 50 and 80 mph), you can ignore everything else.

Click to expand...

That doesn't make logical sense, Hank.


Rolling resistance is still there. Ernie Rogers has done some excellent write ups about decreasing rolling resistance and getting better economy. Many of us have shown switching to 506.01 fuel saving oil has made improvements in economy. These factors don't just dissapear into thin air (pun intended)...they are still very much factored into the overall drag placed on the car.

I notice the weight factor. I too drive around w/o a spare. I don't track fuel economy though...but a lot of my driving is city (35-40%)...and f still equals m*a.

burpod said:

i think city driving weight would be more of a factor than cruising on the highway. i've always worried about reducing weight to improve economy (in my jetta i kept the spare tire/jack in the garage)...

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I am sorry but running around no spare or jack Did you at least go nude I mean the weight of your jack is not more than a decent pair of shoes You could take out the side windows and use rolled plastic
Guys the addition of less than 100 lbs should not make any measurable difference. To save more you could put a little tiny Batt in it and park at the top of a hill to start it that could save another 20 lbs. How about not filling up the washer fluid that 6 pounds or so. Make sure you do your business in th AM before leaving the house and don't eat or drink until you return and won't use the car for the rest of the night. Did you at least remove the seats when no one else should be in the car that day?
Sound silly but driving without a spare and a jack unless you live within walking distance of your travels to save the weight to me is foolish even if you have AAA they can't put on a tire you don't have not to mention that the wait for a truck is often a few hours Sorry to get on a soap box but lets get real shall we.

hey i'm not that nuts about reducing wieght only thing done was removing the tire/jack/etc for about 45lbs. eventually i want to remove the muffler but this isn't purely for weight reasons. for my previous location, type of driving and conditions, i felt it was safe enough to "chance" it (have never had a flat while on the road in my life as far as i can remember). i'm keeping the spare in my gf's tdi. however after seeing my mileage with my car even when it was fully loaded down, i'll probably be putting the spare back in...

Famous last words:

burpod said:

(have never had a flat while on the road in my life as far as i can remember...

Click to expand...

Of course, it's extremely unlikely to happen in northern Minnesota, in the winter.

I drive from Colorado to NC and back seversal times a year . I went 13 years without a flat until last summer in bum f**k Kansas in a contstruction zone I ran over something . Before this the last time I lost a tire on a long trip was in my 85 Jetta in 1990 just outside of Toronto Ont. on a really hot day .

I put the D-nut on ( 50 kph max rated on the 75 mph highway ) and drove to a small town and paid $50 for a used tire . They had me by the B*lls so what could I do . I normally replace the D-nut first chance I get with a full size spare but I had reinsatalled the D-nut to save space in my 85 Jetta a while back and forgaot about it . Will not do that again .

The cars is now parked in NC and first chance I get a real spare is going in the trunk before I start another drive accross the US . There is a big difference in being 30 miles form home and getting a flat and being 2,000 miles form home and getting that same flat .

We can take the best care of our tires but still be taken out by a piece of debree in road without warning . So I would never suggest driving without a spare D-nut or otherwize .

On mpgs being lower because of having extra weight , in my experience a diesel powered car really makes no difference in mpgs . Now on a gasoline powered car a real change in mpgs can be observed with changes in vehicle mass .

I keep the spare in the car, but wondered about removing it for weight savings.

I do keep in the trunk a small hand pump, tire guage (dial type) and a tire plug kit.

I've had maybe three flats in the past 20 years, two of them I drove to the service station, aired up, then found a tire repair shop within an hour or so. The nails were still in the tire so the leak was slow both times. The third time I aired up with the bicycle pump a few times on my way to get the flat fixed. I could have changed the tire and used the spare, or taken a shot at plugging the tire on the roadside also. You do not even have to remove the tire to install a tire.

One thing, If I ever plugged a tire, I would later have it fixed properly--Have it dismounted, install an internal patch, mount and inflate and balance.

--Nate

"You do not even have to remove the tire to install a tire."

You do not even have to remove the tire to install aTire Plug.

--Nate

Not really about TDI's, but it'll give you an idea:

My '01 F350 gets 14-15 mpg unloaded. Weight is around 8500#.

Same F350 pulling a 3500# trailer, 2 Jeeps, a pile load of tool, fuel, beer, etc, gets about 9-10 mpg. Total combined weight is around 21k#.

I'm about to tow w/ a new trailer (a gooseneck wedge), with 3 Jeeps on it, combined weight will be right around 26k#. I expect the mileage to be around 8-9 mpg.

JP

nicklockard said:

That doesn't make logical sense, Hank.


Rolling resistance is still there. Ernie Rogers has done some excellent write ups about decreasing rolling resistance and getting better economy. Many of us have shown switching to 506.01 fuel saving oil has made improvements in economy. These factors don't just dissapear into thin air (pun intended)...they are still very much factored into the overall drag placed on the car.

I notice the weight factor. I too drive around w/o a spare. I don't track fuel economy though...but a lot of my driving is city (35-40%)...and f still equals m*a.

Click to expand...

The point is that at high speeds wind resistance has a much larger effect than rolling resistance. Yes rolling resistance is still there, but you can ignore it.

You might save a couple mpg by getting rid of weight. You can save a lot more though by driving 55 (this speed varies by car) instead of 110.

Fuel

It takes fuel to carry fuel !

sending a program for calculating effects

Interesting discussion--

Here is the formula for where your fuel goes:

F = Cd A 1/2 rho V^2 + Crr M g + Ci 1/2 M V^2

Cd is aerodynamic drag, Cd = 0.30 about for Jettas and Passats
A is the frontal area, about 2 sq.meters
rho is air density, about 1.2 at sea level
V is the car velocity in meters /sec = mph x 0.447
(You have to square V)
Crr is rolling resistance coefficient, Crr = 0.009 for Mich. Energy tires
M is the weight of the car in kilograms = lb /2.2
g is the gravity constant, g = 9.81
Ci is a correction factor for stop-and-go,
Ci = 0 on the highway,
Ci = .0001 about for light city driving
Ci = .0002 about (or more) for heavy bumper to bumper traffic

If you have the right multiplier, you can convert F to gallons per mile.

I use a little program that I put into Excel to evaluate this stuff. It might be on the site here somewhere, sorry I don't know how to post programs yet.

Ernie Rogers

hank miller said:

Don't forget rolling resistance. I can't remember my friction physics, but I know this increases linearly with speed and mass.

(something like f=kmv, where f is friction)

Though I should note that wind resistance increases with the square of speed, and mass has no effect on this. Once wind resistance becomes the dominate factor (generally between 50 and 80 mph), you can ignore everything else. Until wind resistance takes over the faster you go the better, because engine losses are related to RPMs (and gears mean you can keep rpms low), and significant enough that you can get better milage by optimizing that factor (though not significantly).

So if you have a heavy car you can go faster before wind resistance overtakes the other factors. If your car is light your max speed before wind resistance takes over is slower.

The bottom line: your car will physically break long before mass has any significant effect on milage.

Click to expand...

hank miller said:

The point is that at high speeds wind resistance has a much larger effect than rolling resistance. Yes rolling resistance is still there, but you can ignore it.

Click to expand...

So to summarize, you're saying that until 50mph or so, weight and rolling resistance is the biggest detractor from mpg. After 50mph, or say when shifting into 5th gear (for those of us with the proper trans ), wind resistance takes over as the bigger problem.

One could then hypothesize that it would be beneficial to remove your spare and jack in city driving, where you would experience more stop-and-go traffic, and would not be far from home. Highway travel, especially far from home, would be a good time to have a spare in the trunk.

Wow, this is interesting....
Originally I was going to post and say that weight has very little effect on mileage.... now my brain hurts.
I just got my best mileage tank ever, and I had 3 passengers in the car for 90% of the tank, and it was 100% city driving. But, none of my passengers were nude, and I started with a full tank, so who knows how any of these factors effected it.... come to think of it, the vast majority of my passengers were male, if they had been nude, I would have driven faster just to get them out of my car... that would have hurt mileage

Yeah, the thought of four nude men in a car is an unpleasant vision.

You also got this mileage on winterized fuel...hmmmmmmmmmmm

The TDI gods were watching out for you.

Perhaps your TDI is starting to get broke in, and you have even better mileage to look forward to.

cptmox said:

So to summarize, you're saying that until 50mph or so, weight and rolling resistance is the biggest detractor from mpg. After 50mph, or say when shifting into 5th gear (for those of us with the proper trans ), wind resistance takes over as the bigger problem.

One could then hypothesize that it would be beneficial to remove your spare and jack in city driving, where you would experience more stop-and-go traffic, and would not be far from home. Highway travel, especially far from home, would be a good time to have a spare in the trunk.

Click to expand...

Not exactly. My comment is where you sould optimize your efforts.

If your goal is to maximize your mpg for city driving, then removing mass (such as the spare) would help more than something that decreases wind resistance. Less weight[1] is better At slow speed something that reduces wind resistance may cost more (because of mass) than you gain.

For highway driving you will get the same results removing the spare, but you could get much more dramatic results if you found a way to decrease wind resistance. (various fins can do this, but only if engineered correctly)

I do hope that you all recognize that I'm being a little silly. The factors I cite are clearly true, but in the real world there isn't much you can really do about them. Instead of removing the spare from the trunk, go to the gym and remove the spare from around your waist. It will have the same effect, plus it turns the time you are now wasting worrying about if it is worth removing the spare.

[1] For the physicist I am intentionally mixing mass and weight both are important, but to different terms in the equation that someone else helpfully posted.

TornadoRed said:

Famous last words:
Of course, it's extremely unlikely to happen in northern Minnesota, in the winter.

Click to expand...

lol! but in the winter all the snow on the ground covers up any sharp objects that might pop your tire just joking of course.. believe me, i've thought about my "famous last words" in any case, to re-iterate, i keep the spare tire in my gf's golf, my jetta is being garaged for a little bit so it isn't being driven around in northern mn without a spare, so guys don't worry like i said, i'll probably put the spare back in at least in the winter, (when i start to drive it again)... as most of us know, the most effective way to improve mpgs is driving it slow on the highway.

An easy way to check this is to load up dad and the least favorite kid. Fill up and drive until the tank is empty. Drop off the kid where you are, fill up and drive back.

You get rid of the kid 700 miles away and you find out about the weight differential.

cptmox said:

Yeah, the thought of four nude men in a car is an unpleasant vision.

You also got this mileage on winterized fuel...hmmmmmmmmmmm

The TDI gods were watching out for you.

Perhaps your TDI is starting to get broke in, and you have even better mileage to look forward to.

Click to expand...

I revised my driving habits, and I think I found the way to drive for optimum mileage finally. It took 10 tanks, but I think I've got it down pat. I expect 900km's per tank in the city, and it's an automatic, so I think that's doing pretty well.
My last tank beat the EPA window-sticker numbers, and again, it was on winter fuel, so I am pretty impressed.
If I can continue with the good mileage that will be a pleasant change. Up until now I've been averaging 25-26mpg. The last tank was well into the 30's. Same fuel stations, same routes, more passengers than normal, the only thing that changed was my right foot. It seems to have made all the difference.

kasper said:

In airline operations, the tankering of fuel is used often due highly variable prices internationally. We have software programs in dispatch to calculate the savings...and they can be rather substantial. But several factors are different for a car. An airliner can usually carry about 1/3 of it's weight in fuel...so as a percentage of total weight ...this is significant ....whereas a car reduces its total weight as fuel is burned to a much smaller degree. An aircraft would cruise at a lower altitude initially then climb as fuel is burned...on an ocean crossing...you might climb 2 thousand feet every couple of hours...a car can't look for less dense air (less drag). For an aircraft such as an Airbus 310, a rough guide is 3% of the extra fuel weight per hour. eg 10,000 lbs of extra fuel results in burning 300lbs per hr more ...just to carry it.
My experience with the TDI is anything you can do to reduce weight is only marginally helpful....now put a roof rack/bike rack or trailer into the equation and you will certainly see your "Burn" go up.

Good question though!
Cheers
Ted

Click to expand...

For a specific flight the burn correction formula was

ORIG MEL/YMML MELBOURNE INTL........ S37404E144506 ETD 1020Z
DEST PVG/ZSPD PUDONG.................... N31085E121474 ETA 2018Z
ALT1 SHA/ZSSS HONGQIAO................. N31119E121201 ETA 2029Z

ZFW INCR / 1000 .. .. .. X 217 KGS = .. .. .. BURN ADJ
BURN.......82,384kgs / ADJ........... 09.58hr/min 4950Nautical miles

Click to expand...

For those in aviation the call sign is "GTI"

We can add up to 10,000kgs of payload or fuel but after that the flight plan has to be recomputed by a dispatcher.

To get back on topic, fuel consumption is related to weight simply as a function of a higher load on the engine. To a greater extent stop and go driving will suffer a greater penalty due to the longer duration of high loading on the engine during acceleration.

For aircraft the penalty in carrying heavier weights is in the climb, as well as the thrust needed to overcome the increased lift required to support the higher weights, however the fuel used in the climb is often recovered in the decent (usually).

DB