The ratio is the relationship between a vehicle's driveshaft (or propellor shaft) and its wheel axle. For instance, a 4:1 axle ration means the drive shaft turns 4 times for every one time the tires spin. The higher numerically the axle ratio, the greater force that can be applied to the drive wheels for tasks like towing up a grade or pulling a boat out of the water. (See also Final-Drive Ratio
By changing a vehicle's axle ratio, you may change its towing capacity.