Margarita Girl

Had a chat with a motor head about changing the gears in my standard shift from 3.55 to 3.73. He told me since the gain was less than .2, I could get the same results from decreasing the total diameter of my tires. My stock tires have a high profile and I can cut them back on my stock 17's. Since I've got no lowering kit, the 1/2" my car will drop has no effect.

True?

Daniel60

that's why I refused to go to a bigger tire because it will take away horsepower. I think he is correct because the car brenspeed uses on the drag strip has 15 inch mickey thompson dragstrip tires.

Stoenr

You're asking for more wheel well gap. But yes going to a smaller tire will effect the final drive ratio as gears would. Also will have a small effect on ride and handling.

kleistang

its true, it would be easier for your car to move smarller wheels with more ruber, remember that the motion from the differencial goes 1st to the wheels, wheels rotate then the tires, its better for traction also.

Lees07GT

Reducing the tire profile will effectively accomplish the same thing as increasing the gear ratio. It shouldn't affect horespower, however. The stock 17 inch tires are 235/55, I think. If you went to 235/50 for example, it would reduce your overall wheel/tire height by 0.925". This would be a reduction of 3.4% and would effectively change your 3.55 gears to 3.67. It would also cause a 3.4% error in your speedometer (actual 60 would indicate 62). This could be corrected for using a tuner.

22kart22

Horsepower doesnt change. The only thing that happens is you are decreasing the gear ratio between the tranny and the outer circumfrence of the tires... You can accomplish the same thing by dumping tire pressure

CrazyAl

Exactly!

Changing your tires to ones with a different outside diameter (not to be confused with the wheel's diameter) has the same effect as changing your gears. Smaller OD tires are equivalent to numerically higher gears. Larger OD tires are equivalent to numerically lower gears.

As Lees said, changing tires (or gears) does not affect your horsepower, but it does affect "gearing".

I am running 295/45-R18s on the rear in my car, becasue I like the "musclecar" look of the larger diameter tires in the back. Those tires are about 1.5" larger diameter than stock, and they really fill up the wheelwell. To compensate for that I am running 4.30 gears. The combination of the larger tires and those gears are equivalent to running 4.10 gears on stock-size tires.

P Zero

It does affect your horsepower. Increased tire size or diameter will increase your ROTATING MASS. When you increase it it WILL take more power to turn the tires over. They proved it on speed TV with some focus they were building for road courses. I forget the name of the show, but they dyno'd the car with the factory rims and tires then again with a larger set of 17's. They lost something like 4 hp but they improved their overall lap times.
-P.

wingman75

I'm mathmatically declined, so could you figure the difference between the stock 17" and the 18" optionals, on an auto trans car, as far as final ratios is concerned? The 17s should have a slight advantage from a dig, but how much? I'm sure its negligable.

Thanks!

Lees07GT

To be sure, I would need to know what the optional 18 tire size is. But I believe that the overall tire/wheel height is approximately the same with the 18's having a lower profile tire than the 17's. Can you tell me thesize of your tires?