gear ratio

CUDA69 said

Keep one wheel still for an open differential, but you will have to let the moving wheel rotate two complete times. So the the drive shaft will rotate 3.91 times while the moving wheel rotates twice. That would be your open 3.91.

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THIS

Jack up one wheel only, in neutral, and see if you can turn the tire. If you can, use the above procedure.

If you can NOT turn the tire (in neutral) you have a Sure Grip or some other form of limited slip/ locker

So then jack up both wheels and count the driveshaft for ONE revolution of a wheel.