Rollout is Tire Circumference (diameter x pi) divided by final drive ratio.
I typically measure my rollout in inches, so a 57 mm tire is ~2.25" (divide by 25.4)
So for you it'd be (2.25 x 3.14...) / ([100/pinion] x 2.5)
Basically you have to use rollout when your tire diameter changes due to wear or cutting the tires down more. You have to make sure you stay within a +/- .01". So if your target rollout is .95", a .94 or .96 will be ok, but once you exceed that, you'll have to readjust gearing or tire size. Rule of thumb: one tooth on the pinion is equal to .03" for rollout. So if you had a .95" rollout and went up a tooth, you'd have a .98" rollout.
USVTA Member #211
Yokomo BD7 (Mod TC), Corally Phi '07 (VTA)
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