Author Topic: Rolling resistance? (Read 5206 times)

Powjetta · « **on:** January 11, 2009, 10:36:55 pm »

I've got a question for you guys about rolling resistance and any easily done tests for checking this. I just changed my wheels because my wife is driving the car temporarily and I know its easier to steer with the original 175/75/13 and manual steering. I put them back on with 45 psi in front and 35 in the back, and the car seems much faster now and seems to go generally faster on hills, top speed, accelerating, whatever. The wheels I took off were 205/55/15 Fuzions that I guess didn't have enough pressure. The 15 inch tires are marginaly taller, I know.

I have read in car mags about a coast down test, I guess I can put something together but don't want to reinvent anything.

Anybody done this before?

Smokey Eddy · « **Reply #1 on:** January 12, 2009, 12:39:16 am »

Mileage differences.

done.

clbanman · « **Reply #2 on:** January 12, 2009, 02:55:38 am »

There is a 2.33% difference in circumference on the two tire sizes. There can be differences in inertia for a larger diameter tire if for example the rim is lighter/same weight but the tire itself was heavier. (More mass further from the center of the rim.) This would reduce acceleration. The air pressure change can make substantial differences in mileage. My brother had a Subaru Imprezza that he bought new. The first couple weeks he drove it without checking the air pressure in the tires with the assumption that the dealer would have checked it during the PDI he paid for. When he finally checked it, all 4 were at about 22 psi. He said inflating them to the recommended 32 psi added almost 100 km per tank to his driving distance.

TedV · « **Reply #3 on:** January 13, 2009, 09:25:57 am »

google and read how the hypermilers test their modifications like you have already.

The weight or the rim and wider 205 tire will slow the acceleration but wider tire is better for cornering. Grassroots motorsports Magazine did a test several years ago drag racing a stock econobox Honda Civic (low HP). base line was the factory 13" tires and they increaded the rim by 1" keeping the tire the same circumfrence. Each increase in rim size=slower time. Slowest being the 205/?- 17 tire, fastest was the smallest and lightest rim.

I've done the milage test between different tires and also a coasting test on a back road. Yes, skinny and higher pressures roll easier.