P Zero

Its not the size that takes the HP its the WEIGHT.
-P.

wingman75

I guess you are right about them being about the same considering the difference in aspect ratio. That went right over my head. But I did measure my 17 inchers tire width at 26-1/2".

Thanks!

forensicsteve

This is why I am selling my Nitto DRs 315/35/17. They are slightly smaller in diameter than the stock tires, and this givesthe caran effective gear ratio of 4.34. Makes daily driving more difficult and carwould run out of 4th gear in the 1/4. For the track,we're going with 27.6" diamslicks that reduce the ratio to 4.04 but getus through the 1/4 (in theory anyway until next wednesday).

No idea about hp changes though.

Lees07GT

Okay, I found the factorytire sizes in my owner's manual. The 17 is 235/55 and the 18 is 235/50. Doing the math, the 17 total tire height is 27.18". The 18 works out to be 27.25". So the overall size of the 17 and 18 tires/wheels are essentially the same and there would be no noticeable effect to gearing.

modular81

I'm pretty sure the weight of the wheel and tire combo is what causes aincrease or decreasein rwhp on a dyno.

RodeoFlyer

couple things here -

first of all, you are NOT making any more power. your engine does not magically make more horsepower. you are putting it to the ground at a faster rate.

yes, shorter tires change your gear ratio, HOWEVER - you now have a smaller contact area on the ground. A 26" tall tire contacts less rubber to the ground in one revolution than a 28" tall tire. This means that per revolution even though they are the same width the taller tire puts more rubber to the ground. NHRA Stock Eliminator cars are a good example of this. They are only allowed a 9" wide slick, so they run 30" tall ones for a larger contact area. A taller tire will alsohave a larger sidewall to wrinkle better, allowing better traction. You will se NMRA guys running 28x10.5's much more than 26's.

2cokeman

So the stock tire package on the mustang both in 17 and 18 inch should have really no difference??

CrazyAl

There is no appreciable difference between the two for gearing.

However, the 18" wheel-tire combo is slightly heaver and it also has higher rotational inertia not just from the weight but becasue the radius of the wheel is higher. Thus the 18" combo should be slightly slower than the 17" setup...but it is very minor.