Overall tire diameter times pi divided by gear ratio (spur divided by pinion) gives roll out. Your known variables are tire diameter, pi, and spur. We need to decide on a roll-out and choose a pinion.

I've not done any real "brushed" mod racing, preferring to run brushless in mod. I gear my 19T and stock in the mid-high 40's for roll-out, but when I'm trying something new I prefer to be a little conservative and sneak up on the right combo. So let's say 40mm is the desired roll-out, just for argument's sake...if anyone disagrees please speak right up!

So, 48mm diameter times pi (3.14159, or 22/7, your choice) gives a circumference of approx 150.8mm. Now we need to find what one would divide 150.8 by to give 40 (remember that crazy Commutative Property of Multiplication? It works for division too!)...the answer is to divide 150.8 BY 40, which yields 3.77. Similarly, if we divide 74 by 3.77 we get a pinion of 19.62. That .62 tooth plays HELL with spur gear life, so you'll find that 19 teeth is a bit under 40mm rollout and 20 teeth is a bit above.

Now...during your race day you need to monitor the tire diameter. As the diameter decreases you need to increase the pinon tooth count to compensate and hold the rollout at the desired dimension.

Another "hint"...use 64P gears for 1/12. Not only are they more efficient, they'll also give closer gear selections as a tooth either way on, say, a 28-tooth pinion is a much finer increment of adjustment than a tooth either way on a 20-tooth pinion.

hth. Hopefully someone will point to the site(s) that have gearing calculators. I use the Excell-based one Blake posted some time back and have it downloaded to my computer.

Scottrik