The rearend gear ratio refers to the relationship between the driven gear (ring) and the drive gear (pinion), and is easily calculated by dividing the ring gear tooth count by the drive gear tooth count. For example, if the ring gear has 41 teeth and the drive gear has 10 teeth, the correct gear ratio would be 4.10:1.

The gear ratio between the transmission and the wheels ultimately determines how an engine’s power is used to turn the wheels. When we talk about the rear end gear ratio, we’re referring to the relationship between the number of teeth on the ring gear and the pinion gear within the differential of a vehicle’s rear axle.

When you hear car drivers refer to numbers – such as 3.08:1, 3.73:1, or even 4.10:1 – they discuss the ring-and-pinion gears’ ratio within the rear axle. This ratio refers to the number of teeth on the ring – i.e., driven gear – divided by the number of teeth on the pinion – i.e., the drive gear.