High gears?

Dubob said
If it's an open differential, you'll need to keep one tire on the ground then count the revolutions as the spider gears will allow the other tire to turn and not be accurate.

Right as far as it goes, read onswards

replicaracer43 said
This is wrong, you need to turn both tires at the SAME TIME, 1 revolution (might require a helper) and count the driveshaft rotation.

RustyRatRod said
THIS is correct. ^^^^^

NO, no NO!!! Here's how you "do" an open rear end, and it's easy!!!

You DO leave one wheel on the ground!!!

And you go around and turn the jacked up wheel TWO revolutions while counting driveshaft turns. That will directly give you the ratio, IE 3 1/2 "plus a bit" is 3.55, etc

If it's limited slip (suregrip) then you jack up both wheels, and turn either wheel ONE turn, counting the shaft