Volkswagen Passat Forum banner

1 - 20 of 21 Posts

·
Registered
Joined
·
1,763 Posts
Discussion Starter #1
So I've noticed that my MPG is down after I upgraded from 15 to 17 inch wheels...as far as the computer is concerned at least.

Did anyone else see this after getting bigger wheels?

Since there is more rubber on the road, i figured it would throw the odomoter and other gauges off since i am going faster with less RPMs...

I realize that the real mpg may be the same, but I would like my speedo, tach, odometer to be as accurate as possible cause don't want a ticket for going 85 when i thought I was going 80, etc.. Has anyone reprogrammed their speedo or reset it somehow to compensate for larger wheels?
 

·
Registered
Joined
·
11,541 Posts
Your 17's, what series tires are you running?

Look at http://www.miata.net/garage/tirecalc.html to see what effects you might be experiencing. Understand that if you upsized your wheels and downsized your tires correctly, the speedo change is usually not significant. However going to 17's will be less efficent at the pump with increased unsprung weight and more rolling resistance.
 

·
Registered
Joined
·
580 Posts
Ok, I'm guessing your old tires were 195-65R15?? What are the new tires?


If you are using 225-45R17 or 205-50R17 your speedometer should be dead on accurate (within 1 percent). You are probably experiencing a real MPG drop from wider tires that have more rolling resistance (think of the difference between riding a bike with a thin road bike tire and a mudder tire) and the 17inch wheels probably have more inertia, meanining that you need more gas (energy) to accelerate to the same speed as before (this is why when people change from 15s to 19s their car feels sluggish).

Anyway if its a few mpg (like say combined average of 27 to new average of 25), then its probably a real reducing in fuel economy and not speedo calibration.
 

·
Registered
Joined
·
1,396 Posts
Assuming you're using the "proper" 225/45-17, your speedometer should read around 0.4% higher than it did before (already a couple % higher than "actual").

I'm running 17x8s compared to my old 15x6s (stock steelies), and have lost a tiny bit of mileage, though it depends on the situation. City driving doesn't seem to have gone anywhere, but I think my highway mileage has dropped (though it could also be gas/plugs).
 

·
Registered
Joined
·
234 Posts
welcome to the world of increased unsprung weight and increased rolling resistance.
Bigger wheels and tires don't just cost you upfront. It takes alot more torque to turn those heavier donuts and more braking to slow them.
It's a different ballgame.
 

·
Registered
Joined
·
1,763 Posts
Discussion Starter #6
The old ones i believe were 195/65 15 and the new are 235/45 17...

On my first tank of gas the computer MPG went from 30ish to around 27ish...
 

·
Registered
Joined
·
1,763 Posts
Discussion Starter #10
The other question this brings up is this:

Was the Speedo correct from the factory with the 15s? (it always seemed like the radar gun was slower than my odometer...)



Good point about the pressure. I may need to go to the 37-40psi range for better MPG on my 70 mile commute to increase MPG...

What PSI do you run on your 17 and 18's?
 

·
Registered
Joined
·
6,172 Posts
LoneStarPoke said:
Was the Speedo correct from the factory with the 15s? (it always seemed like the radar gun was slower than my odometer...)
Our analog speedo is approx 3mph too slow; however, the digital data is accurate -- set your cruise control on a flat strech of highway and reset the digital readout while viewing average speed: it will show 3 mph slower than the speedo. You could also use a hand-held gps to determine your actual speed.
 

·
Registered
Joined
·
1,428 Posts
Exactly, more rolling resistance (from a wider tread) + more rotating mass (from a heaver wheel) = less MPG. I noticed a 4mpg drop after a +3 plus size. After chipping I gained all the MPG back. I think the added torque and horsepower earlier in the RPM range really help spin those heavy wheels.
 

·
Registered
Joined
·
1,763 Posts
Discussion Starter #13
B5point5 said:
Exactly, more rolling resistance (from a wider tread) + more rotating mass (from a heaver wheel) = less MPG. I noticed a 4mpg drop after a +3 plus size. After chipping I gained all the MPG back. I think the added torque and horsepower earlier in the RPM range really help spin those heavy wheels.
Sounds like a good excuse to get chipped...wonder if the wife will buy into it... :cry:
 

·
Registered
Joined
·
1,132 Posts
pete1 said:
LoneStarPoke said:
Was the Speedo correct from the factory with the 15s? (it always seemed like the radar gun was slower than my odometer...)
Our analog speedo is approx 3mph too slow; however, the digital data is accurate -- set your cruise control on a flat strech of highway and reset the digital readout while viewing average speed: it will show 3 mph slower than the speedo. You could also use a hand-held gps to determine your actual speed.
A minor detail - the analog speedo is three miles per hour too fast for stock diameter tires, and the computer and odometer are accurate for stock size tires. Stock is either 24.9" or 25" per TireRack. 235/45 17s vary by manuafacturer. A mich pilot is 25.1" a Potenza 030 is 25.5" - about 2% bigger than stock. All of this is so close, noone probably cares... :nervous: but I love details.
 

·
Registered
Joined
·
450 Posts
Your MPG should not drop too much. I forgot what I was getting before. But after upgrading from the stock 15 to the extremely heavy S4 17's, I am still averaging 26mpg. :lol:
 

·
Registered
Joined
·
1,003 Posts
2 things that are possibly factors leading to worsened MPG.

Firstly your 17"s are presumably heavier

Secondly 235/45/17s are going to cost more fuel than 225/45/17 - easy way to think of it is simply you have a larger surface area and the torque is more spread out and more force/power/gas needed to bring your car to move.

Lets talk baby-physics:
F = mu * W
where mu is the friction coefficient of the tyre, W is weight of car.
Larger F (driving/engine force) is required to move the car if mu is greater.
 

·
Registered
Joined
·
902 Posts
GLX

Stock 15" Adelades: 28-29 Highway
22-24 City

17" Charlestons: 24-25 Highway
19-21 City

225/45/17 - ContiExtremes.

I noticed it right away. Mixed highway/city driving I would get upwards of 350 miles a tank. Now I get to about 300. :???: :mad: :( :cry:
 

·
Registered
Joined
·
156 Posts
This is valuable info - especially since I am about 3 hours away from calling up to order some new rims and tires. I had already decided to go with 18's, but after reading this I am wondering if I should just get some new 15's! :(

I cannot handle torque loss...... :thumbdow:
 

·
Registered
Joined
·
6,172 Posts
ganseg said:
A minor detail - the analog speedo is accurate for stock diameter tires
I disagree - it reads 3mph fast for stock diam., but we're off-topic.
 

·
Registered
Joined
·
26,811 Posts
Does it read 3mph off all the time or is a specific % ? I thought it was a percentage not a preset.
I dropped 2-3 mpg hwy from stock 15's to 215/55/16's.
 
1 - 20 of 21 Posts
Top