mpg with bigger rims

[marinemonkey]

has anyone gone from the 15'' steelies to a 17'' rim that watches there mpg notice a big difference? also how much does a 15'' steel rim weigh i have found mixed answers ranging from 15-22 lbs thanks for the help guys

[blindsnyper]

a heavy 17" alloy wheel is 19+ lbs light ones can be down to 15lbs, but the wider tires will make a bigger difference in weight.

[marinemonkey]

17x7 is what im looking at any help?

i know you guys change rims all the time has anyone ever weighed a 15x6 steelie?

[Promise Land]

my steelies were around 16lbs and my Spec-V wheels were about 18.5lbs. You won't notice much of a difference in MPG (I'm still getting ~32 with the auto trans) but your speedo will read ~ 3-5% high. 60mph is really about 56mph.

[kc5f]

You won't notice much of a difference in MPG (I'm still getting ~32 with the auto trans) but your speedo will read ~ 3-5% high. 60mph is really about 56mph.

If your speedometer reads high, so will your odometer. So when your car thinks it's gone those 60 miles in an hour and it's really only gone 56 miles, your measured 32 mpg in the real world is actually only 29.9 mpg. Suddenly it doesn't sound so good...

[marinemonkey]

ok so if i put the 215/45/17 its close to the same size tire thats on it my speedo and mpg will be off

[kc5f]

The diameter of the tire is all that matters to the speedometer and odometer. They expect the tire to rotate at a certain rpm for a certain speed. When the diameter is different than they're calibrated for, then the tire rotating the same number of times covers a different distance.

I would have thought a larger tire would cover more distance per rotation, giving a lower speedometer reading and odometer distance for the same true speed. That would show as worse mpg than you'd really be getting. I just used Promise Land's figures since he's got experience with it...

[marinemonkey]

can you re-calibrate the car since it is all electrical? if so who and where could i get this done? thanks for all the help guys

[kc5f]

You'd think it would be something at least a dealer could do, but after searching a bit it looks like they don't usually even offer it. This link can help you see how much of a difference you're actually looking at. But the only ways I could find to actually change the calibration were either for trucks or paying someone $$$ to do it.

For the tires you listed, it doesn't sound like much to worry about. The calculator above gives these results:

Size - Sidewall - Radius - Diameter - Circumference - Revs/Mile - Difference
185/65-15 - 4.7in - 12.2in - 24.5in - 76.9in - 824
215/45-17 - 3.8in - 12.3in - 24.6in - 77.3in - 819 - 0.6%

When your odometer shows 100k miles you'll have actually gone 610 miles further. When your speedometer shows 60mph you'll actually be going 60.37mph.

[marinemonkey]

thanks that link helped a lot