A differential axle is an axle that incorporates a set of drive gears in the axle in the same manner as a rear-wheel drive vehicle uses a gear-equipped axle to power the vehicle. Identified by the large rounded center section, often called a pumpkin, the differential axle contains a ring and pinion gear set designed to multiply the input power of the engine and give the vehicle greater power. Conversely, a non-geared axle, or a straight axle, as they are often called, is a straight axle with a wheel hub mounted on each end, similar to a trailer axle.
The purpose for a differential axle is to multiply the amount of power that is inputted and create more power at the output end of the axle. Using a small gear driving a larger gear, the gear ratio can be arrived at by doing simple mathematics. The small pinion gear receives power from the transmission, which is putting out the same number of revolutions as the engine crankshaft in a non-overdrive transmission application. This means that every time the engine crankshaft turns one full turn, the drive shaft coming out of the transmission also makes one full revolution. By counting the teeth on the larger ring gear and dividing that number by the number of teeth on the small pinion gear, the final gear ratio — or the number of times that the pinion gear and drive shaft must revolve in order to turn the ring gear and axle one complete turn — can be calculated.
Knowing the differential axle gear ratio is critical in deciphering many vehicle functions where it is necessary to apply mathematical equations and certain formulas. The speed of any given vehicle can be figured accurately by calculating the differential axle gear ratio and the tire diameter, along with the engine's crankshaft revolutions per minute (RPM). A tire will travel the same amount of linear-distance as the tire measures in height for each complete revolution of the tire.
For competition vehicles, a variety of differential axle gear sets may be taken to a track to allow for fine-tuning of the vehicle for changing track conditions. The differential axle in a passenger vehicle has undergone many hours of research to arrive at the most beneficial gear set. The factory gear selection in an original equipment differential axle is often a compromise of performance and fuel mileage. Any change in the original factory differential axle is likely to void the drive train warranty of the vehicle.