The ratio between the pinion and ring gears in the differential that multiply the torque provided by the engine. It is the number of drive line revolutions required to turn the axle one time. As an example, with a 4.10:1 axle the drive line turns 4.1 times for each full axle revolution. The higher the number, the more torque and thus more towing power. However, the higher the number also means less speed.