Thread: Tire Diameter

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I was at the track the other day and the question of tire size came up. I am trying to find some concrete information on tire diameter in the apspect of performance. It seems that for the most part the diameter of the tires being raced with stay relatively close together. Rim size might change, but the outside dimensions stay the same. I see bigger tire diameter on the funny-cars, but not much else. My compadre guns a 71 Dart 440, he was running 4.56s I believe, but he found that 4.10 we giving him better results in the quarter mile (his rpms were ending higher than his powerband was). Could a set of larger diameter tires help this situation? When does tire height become a factor, and when is a tire just too big. I have gotten an earfull about the depth, thickness and about wall height, but height was always a bit sketchy? And lastly, in a normal perfomance machine, if I wanted to improve my mpg, could I just get a set of large wheels, thataway I got track tires, and street tires? Any information would be great (It would be easier than trying to swap the top half of my engine every time I wanted to race) Thanks alot.

Originally posted by travisgus
And lastly, in a normal perfomance machine, if I wanted to improve my mpg, could I just get a set of large wheels, thataway I got track tires, and street tires? Any information would be great (It would be easier than trying to swap the top half of my engine every time I wanted to race) Thanks alot.

Welcome travisgus
Changing tire diameter has the same effect as changing gear ratio. A 33'' tire will move about 103" per 1 revolution. A 26" tall tire will only move around 82" per rev. Back in my old racin' dayz' I had a set of indy style tires that where a good 6" shorter than my street tires. They made a big difference on an 1/8 mile track.

Here's two different ways to see what effect tire diameter has;

http://users.erols.com/srweiss/calcrpm.htm

http://www.metropartsmarket.com/rearend/ratio.html