If it's a non posi,jack up one rear tire off the ground,rotate the tire one full turn while counting how many turns the driveshaft make.
Lets say the driveshaft turns a little over 1.5 turns,you would multply it by 2 which will be approx 308's
With a posi unit,,,jack up both tires off the ground,rotate tires one full turn & count how many turns the driveshaft,it will be the exact ratio,so if your driveshaft turns,say a little over 4 turns,most likely it will be 410's
Hope this makes sense.
MBHD
