What Does a Differential Gear Ratio Mean?
A differential gear ratio describes the relationship between driveshaft rotation and axle rotation. A 3.73:1 ratio, for example, means the pinion gear and driveshaft turn approximately 3.73 times for each complete rotation of the ring gear and axle.
Differential ratios are commonly discussed when identifying an unknown axle, replacing gears, changing tire sizes, modifying a vehicle for towing or performance, or comparing different drivetrain combinations.
Higher and Lower Numerical Ratios
A numerically higher ratio, such as 4.10 compared with 3.42, provides more torque multiplication at the axle. All else being equal, it also causes the engine to turn faster at a given road speed. This is often referred to as shorter gearing.
A numerically lower ratio provides less torque multiplication and lower engine RPM at the same road speed. This is commonly called taller gearing.
Neither is automatically better. The appropriate ratio depends on the vehicle, transmission, tire size, engine, intended use, and other drivetrain factors.
Identifying an Unknown Axle Ratio
If the differential is apart, counting the teeth on the ring gear and pinion provides a direct way to determine the ratio. Some gear sets may also have identifying numbers or tooth counts stamped into them.
When the gears aren't accessible, driveshaft and wheel rotations can provide a useful estimate. Because open differentials behave differently when only one wheel is raised, it's important to use the testing method that matches the type of test being performed.
Rotation tests are estimates rather than exact identification. Closely spaced ratios can be difficult to distinguish if the rotations aren't counted accurately.
Gear Ratio Is Only Part of the Drivetrain
The differential ratio by itself doesn't determine vehicle speed or engine RPM. Transmission gear ratio and tire diameter also affect the overall relationship between engine speed and road speed.
Changing tire diameter can noticeably alter how an existing axle ratio behaves, which is one reason differential gearing is often reconsidered when substantially larger or smaller tires are installed.