Originally Posted by AutoGear
you can put your trans in 1st gear, turn the input 10 times, count the number of times the output turns. If it turns 25 times, its an M20 (2.52 1st gear). If it turns 22 times, its an M21 or M22 (2.20 1st). If its an M21 or M22, you'll have to remove the sidecover and look at the helix angle of the gears to decipher what you have.
This needs to be corrected to
"Turn the OUTPUT 10 times and count how many times the INPUT turns. 22 times around on the INPUT would signify an M21 or M22 and 25 times would mean its an M20 or aftermarket ratio M22W."
Thanks to Cobalt327 for catching my error. I meant to correct this awhile ago, but haven't been online much.