Work out an axle ratio from ring and pinion tooth counts, combine a transmission gear with an axle ratio for the overall ratio, and estimate engine RPM at a given road speed.
Axle ratio from tooth counts
Overall ratio
Engine RPM from road speed
The axle ratio is the ring gear tooth count divided by the pinion tooth count: 41 ring teeth over 10 pinion teeth gives a 4.10 ratio, meaning the pinion and driveshaft turn 4.10 times for each wheel revolution. The overall ratio multiplies the transmission gear in use by the axle ratio, so a 0.75 overdrive gear with a 3.73 axle gives an overall 2.80. Engine RPM at road speed uses the standard approximation RPM equals mph multiplied by the overall ratio multiplied by 336, divided by the tire diameter in inches; 336 is the constant that folds in the unit conversions. The RPM figure is an estimate: it assumes a locked torque converter or a manual gearbox, and actual tire diameter under load is slightly less than the unloaded figure.
What does a 3.73 axle ratio mean? The driveshaft turns 3.73 times for every one turn of the wheels. A higher number gives stronger multiplication for acceleration and pulling, at the cost of higher engine RPM at cruising speed; a lower number does the opposite.
Where does the 336 constant come from? It combines the conversions in the RPM formula: minutes per hour, inches per mile and the relationship between tire diameter and circumference. RPM equals mph times overall ratio times 336 divided by tire diameter in inches.
Why might actual RPM differ from the estimate? The formula assumes the tire rolls at its stated diameter and the clutch or torque converter is locked. Loaded tire diameter is slightly smaller, converter slip adds RPM, and speedometer readings are themselves approximate, so treat the result as an estimate.