A gear ratio compares the input and output of a gear set, showing how it trades speed for torque.
Where
| Driven teeth | Teeth on the output (driven) gear |
| Drive teeth | Teeth on the input (drive) gear |
Worked example
A 40-tooth gear driven by a 12-tooth gear gives 40 ÷ 12 = 3.33:1 — the output turns once for every 3.33 input turns, with 3.33 times the torque.
FAQ
What does a numerically higher gear ratio mean for performance? A higher numerical ratio multiplies engine torque more at the wheels, improving acceleration and towing capability, at the cost of higher engine RPM, and typically worse fuel economy, at a given road speed.
Does changing gear ratio affect speedometer accuracy? Yes, since a speedometer’s calibration assumes a specific gear ratio and tire size. Changing the rear gear ratio without recalibrating the speedometer causes it to read incorrectly relative to actual road speed.
Use the Gear Ratio calculator in the automotive calculators, and read How to Calculate Gear Ratio.