Increasing Gas Mileage With Taller Tires? Options

[joyworks]

One of the few things that I dislike about my '99 Celica Convertible is that the gas mileage isn't as good as it could be. The 1997-2001 Camry, which had the same engine, got 3 mpg better on the highway with a 5-speed than my Celica does with a 5-speed, and the Camry weighed about 300 lbs more! I can only attribute this to the high revs of my Celica's 5th gear. So, my question is, can I achieve better fuel economy with a larger diameter wheel/tire combination? I know it would throw my speedometer off a bit, but to save fuel, it would be worth it to me. On that same note, what would would the tallest tire that I could put on a 16" rim and not suffer from any fender rubbing? What about a 17" rim? Thanks for your help!

[dgbldr]

No, no and no.

1. No, you can't attribute the difference between your Celica and Camry to shorter gearing only. You can also attribute it to:
a. "Measurement" error. Unless you have a chassis dyno and can run a precisely controlled driving cycle on both cars, your "measured" fuel economy is just a rough guess.
b. As noted, state of tune, tire type and inflation, etc.
c. Convertibles are generally not as aerodynamic as coupes or even sedans, but that only comes in play with extended high speed driving, like hours on the interstate.
All in all, #a trumps everything else.

2. No, taller tires (within reason) do not add significant weight nor "rotating mass". We're not talking Indy cars here guys. Take a couple of tires, one 205/55 and one 205/65 and weigh them. Less than the difference between a skinny driver and an average one.

3. No, taller tires (again within reason, like one inch larger diameter) will not make a 3 mpg difference. If the taller tire is also wider, the extra drag trumps the extra diameter so you end up worse. What you want is skinnier, higher inflation pressure and low rolling resistance rubber compound.

4. No, increasing the ride height by an inch will not make a significant difference. Again, aerodynamics play a minor role unless you do most of your driving on the Interstate, at speeds significantly over 60.

This post has been edited by dgbldr: Oct 23, 2008 - 4:02 PM