Measure your actual tire height with a straight edge of some sort, round up higher if it's an off number because the tire will be higher when driving, don't go by the tire manufacturer claims as they are always wrong and you wheel width will change the height, scout around because some places sell the driven gears for $15 and some $5 for the same product.
You need to know the tire rpm, revolutions per mile, to find that you divide the tire height by 20168
ex 20168/26" = 775
then multiply the drive teeth (found by tranny, most muncies are 8) X axle ratio X tire rpm
