First thing you do is look in the glove box door and see if there is an build sheet glued in there. If not mark the drive shaft and the rear wheel and turn the drive shaft til the rear wheel goes around once. Counting how many turns and parts of a turn it takes. If it takes 3-1/2 turns of the drive shaft then you have a 3.55 or 4 plus a bit would be a 4.11. Make sure both rear wheels are turning together or your figures will be wrong.
