With the open diff., you have to turn the wheel (2) times and count the # of revolutions of the driveshaft. If you turn the wheel (4) times, count the # of revolutions of the driveline; then divide that number by two . This will tell you if you have 4.10 or 3.90 for sure.
If it's a 4.10, you would turn the wheel two times and the driveshaft would go around 4 times + a little more...........there is no 4.00 for GM.
There was a 3.90 (rare or custom made only). That's the one I would want.
