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. For 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, higher axle ration also mean less speed.