it's an open differential, block one wheel from turning then turn the other exactly one revolution while counting the revolutions of the driveshaft. Multiply the revolutions of the driveshaft by 2 and that's your ratio. In other words if the driveshaft turns slightly over 2 turns it's a 4:10 ratio. If it turns 1-3/4 times it's a 3.55 ratio. If it turns 1-5/8 times it's a 3.23 ratio, etc.
