This might need to be moved into the racing section, but i figured theres alot of smart people in here and alot of tire questions here already so it may be better off. What is the advantage or disadvantage of changing the stock tire size in relation to top speed and acceleration. I have heard many different theories and so on, and have been at several different tracks and seen different set ups out perform others (mostly circle track as im a dirt stock car guy). This is what ive been "enlightened" to so far:

By raising the size of the tire you are increasing the amount of recipicating weight that you have to move with the power the engine is making. Thus you would assume (atleast i have) that this would slow down your ability to accelerate as quickly as a vehicle with the same power output and smaller tires. Now here is the part that has got me in a twist.... its called IPR. I believe it stands for Inches Per Revolution. If two cars, identical cars, same hp, same gear ratio (trans and rear) diff size rear tires, will run two diff times. I cant quite grasp the inches per revolution concept, as i would tend to think that the more inches per revolution the car travels the faster the car would be... example, further distance for the same work. However i have seen big tires get destroyed by lil tires and vise versa. I also know that smaller tires will net a higher useable rpm (at the track) but once again less distance traveled per amount of work. So can someone smarter than me please explain this to me in a way that i can understand and make use of. thanks.

 

Smaller tire = lower top speed, faster acceleration.
Taller tire = higher top speed, slower acceleration.

Think of the tire diameter like a gear ratio.

 

Is this for drag racing?

The tire diameter changes the final drive of the car. That is 2 identical cars with different diameter tires are essentially geared differently. IPR is just a way to measure that difference.

If you have an N/A the smaller diameter tires the better.

It matters less with a turbo, because you have more power and torque to pull the taller final drive that a taller tire gives you.

I used to have 245/45/17 tires in the rear. The car felt kind of lazy getting up through the gears. I switched to a 235/40/17 which are an inch smaller in diameter and the car got through the gears much better. It was a very noticeable improvement.

 

if its road racing, look in the rules most of the time we're running 225/45/15's....

on my car i went from 225/45/16's to 205/50/15's, and it feels like i added 50hp