Originally Posted by liljohn1064

I just want to see if this holds water.

P = Pitch
Diameter of the Spur = D, Number of teeth on the Spur = N.
diameter of the Pinion = d, Number of teeth on the pinion = n.

D = N/P for the working diameter of the Spur
d = n/P for the working diameter of the Pinion

Radius = D/2 or d/2 for Spur and pinion

My internal ratio for my car is 2.0588.

FDR = (Spur/Pinion) * 2.0588

If the distance between my spur gear center and the center of my motor shaft is 1.125 inches, I need a pinion and spur that the combined working radii are equal to 1.125 inches +/- the extra 1/8 inch my motor can still be adjusted. I calculate the the radius of the Spur, the radius of the pinion and add them together. My final number should end up between 1 and 1.25 inches. In my case I also had to end up with an FDR of 3.4 to match my 17.5 brushless.

so...

((93/64)/2) + ((56/64)/2)) = 1.164" which is between 1 and 1.25 inches.

and

(93/56) x 2.0588 = 3.419 FDR

Seem reasonable?