A differential will transfer the power to the wheel with the least amount of traction,Both wheels on the ground the power is applied equally to both wheels. Now lift one wheel off the ground ,while leaving the other on the ground all the power is transferd to the wheel in the air.and will turn twice as many times.
So when you turn the wheel,lets say 20 times and count the turns on the pinion shaft,you have to divide by 10(or half as many times as you turned the Wheel) to get the gear ratio.
