I guess that's where I'm confused. Except when substantially modded, all of our cars are very similar so I would assume that, given the same conditions, our mpg vs. mph data would also be similar and track each other rather than converge or diverge. If the power required to overcome drag proportionately increases with speed, shouldn't the data curve at higher speeds steepen rather than flatten out? Thanks again.
I was thinking the Portos were gonna hurt me, but my understanding was that they provide more inertia for city driving, but doesn't matter as much when at a constant speed (like hwy). Also the larger diameter means fewer rotations and should help right?
2 points.
No, the slope (tangent) of the mpg vs mph curve should flatten. If slope increased or got steeper with speed, that would imply that at some speed (mph) we would get zero mpg and the curve would go negative. Of course that doesn't happen. There will be a speed (terminal velocity) where we cannot continue to accelerate as we do not have enough horsepower to overcome drag and other forces. Even at this speed we will get a positve mpg.
You'll should get a litte more as the 2010 manual is geared quite a bit higher than the DSG. But that's highway only, no a/c, flat roads, no active regeneration, tire pressure higher than normal, and probably summer fuel. Did I leave out an caveats? YOUR MPG MAY BE MORE OR LESS.So what you are saying is if I purchase a 2010 6spd Jetta, and can make myself stay 65mph, I shouldnt have a problem hitting 48mpg on the highway
this makes me happy.
again, this is a GREAT thread
So what you are saying is if I purchase a 2010 6 MT Jetta, and can make myself
stay 65 mph , I shouldn't have a problem hitting 48 mpg on the highway
this makes me happy. again, this is a GREAT thread
Thanks for taking the time to explain, Plus 3. I was looking at the descent of the curve past 50 mph where it does steepen and extrapolating it to the higher mphs where drag becomes an increasingly important factor. Graphs of actual data don't lie. Must be that engine efficiency improves at higher rpms as you seem to be suggesting. Someone else was inquiring about mpgs at high mphs in another thread. The answer is right there in the Auto Bild magazine graph. Thanks for providing the link. Neat stuff!I do want to make another point on the mpg vs mph curve. I did make test run at 40 and 45 mph in 6th gear. What I found was that at 45 mph the MPG were slightly lower than at 50 and still lower at 40 mph. So the mpg curve peaks somewhere around 50 mph and then falls when decreasing speed. This is likely due to running below the higher efficiency points on the BFSC map. At 40 and 45 mph, the rpm is 6th gear is 1289 and 1450, respectively.
Here's my actual data and a smoothed curve.
When I have some time I MAY do the calculations as shown here and compute the theoretical fuel economy.
Again, note the shape of the curve in the Theoretical Mileage graph.
That's why I said "may" do it.
As whitedog said way back then:
"My head just 'splodeded"
In my words, that's way over my head!
D
What is important is "real world" fuel economy not theoretical. But what it would do is cast serious doubt on the "exagerated" fuel economy claims we see. IIRC, I remember someone claming 50 mpg at 80 mph with a common rail TDI.
That's why I said "may" do it.
Derrel,
First:
Yes, but that change was not readily readable at first glance.
Review my Fuelly entries being April 27, 2010. Perhaps you may see a pattern?
Our JSW was new, and not nearly broken it yet at approximately 4,400 miles.
Took some time before I was fully aware of what was actually happening.
GY says a four percent improvement in F E, and I think that is about right.
I am getting more than that, but how much of the increases I am seeing
are from the LRR GY tires and how much improvement in F E is from
going to a taller tire which decreased the RPMs?
Second:
Concerning the question about going to a bigger (or different) diameter tire and not being
able to have your MFD, Speedometer and Odometer be correct, VW has seen fit to
provide us with an adjustment. This adjustment must be done using a VAG-COM!
Very thoughtful and considerate of VW to provide such a feature, don't you think?
In my case, the DSG JSW was set from the factory with a '7'.
For it to be exactly the same as it was prior to changing tires, my friend, using his VAG-COM,
discovered that the adjustment needed to be changed from the stock setting of '7' to a '3'.
It now reads and shows everything accurately just as it did before I upsized the tires!
Questions and/or comments are welcomed.
D
No DSG struggling with the larger size is good to hear.
It is indeed an excellent tread!
I haven't noticed any difference or that it seems to be struggling, if that's what you mean?
In fact, when making a customary California Second Gear rolling stop as in almost stopping at
a stop sigh and rolling through it at one or two mph, it actually moves away in second gear!
When choosing the larger tires, I weighed everything including the diameters and the
different weights of the different sized tires, and because I mainly was doing this to
raise the overall gearing, I wanted to go as large as I could within reason.
In fact, I am considering going even larger now, as in the GY Assurance
Fuel Max tires in size 215/65-16s.
Their tread is actually narrower than the 205/65-16s I have mounted now.
There is room I think? This change would give me another .5 inches in diameter
as these tires measure 27 inch total diameter. Revs are only 774 per mile!
What do you think?
Yes, I did consider the smaller 205/60-16s, but mainly I was after the largest
diameter I could
use, and besides that, the 205/65-16s actually listed for less money!
You must be careful here however, as some of the other brands in
these sizes can be much heavier than you might want to run!
I considered only the GY Fuel Max tires as some of the other brands such as
Michelins weigh as much as 25 pounds, and I didn't want to do that!
I never thought of any strain that going to an overall taller gearing might have on the DSG
transmission. I am glad that you and Oilhammer have brought that up.
It seems like it works so easily and is not straining at all.
I guess time and mileage will indeed tell if I am actually hurting anything!
As a side-note question: Is the same clutch used for both First gear and Reverse?
I read it somewhere, but I am too tired this late to search for the answer!
D
I guess that depends on how much clearance you are getting with the 205/65-16s and whether there is additional room.
In fact, I am considering going even larger now, as in the GY Assurance
Fuel Max tires in size 215/65-16s.
Their tread is actually narrower than the 205/65-16s I have mounted now.
There is room I think? This change would give me another .5 inches in diameter
as these tires measure 27 inch total diameter. Revs are only 774 per mile!
What do you think?
D
Have you considered the clearance with 205/65-16s when using tire chains? I will be buying a set of class S chains for the oem tires.
Mine seems to have plenty of room for me to go larger.
I would be only going .25 inches more diameter on each side of the tire.
My diameter now is 26.5 inches. I would be going to 27 inches with the larger size.
As far as risking a violation of the VW warranty, I never thought about that.
I'm too lazy to dig into that question in the owners manual.
I will simply ask my friend at the dealership for his educated opinion.
D
Mythbusters did a show on drafting versus mpg a couple of years back. IIRC, to see substanial gains in mpg with the car they were using, they had to be within 15 feet of the semi trailer. I.E. NO REACTION TIME!
If I draft which is seldom (I do not do it often) as being a retired professional
driver, I know full well all about reaction times and distances travel
per second, etc., and know that it can be and is dangerous.
Try less than one car length and get your instantaneous readings and then back off.
When you follow really closely behind, you are almost in the trucks vacuum,
and should see reading much higher than you are reporting.
D
You might want to check the warrenty issue with tire chains. I know I've had a couple of cars in the past that stated the anti-corrosion warrenty was void if you ran chains. I also had a problem with a chain slipping and almost pulling out the brake line. It did catch the parking brake line and pulled it enough that the brake locked up.
Clearance specs for chains are in the 2010 JSW owner's manual, so I assume that the warranty would not be compromised. You are right to be concerned about chains slipping and causing damage. I have heard of damage as high as $700 from chain damage (quoting my mechanic friend).