Gear ratio calculator

Final drive = gear ratio × axle ratio. Speed (km/h) = (RPM × tire circumference) ÷ (final drive × 16.667). Useful for axle-swap planning and predicting RPM at cruise.

Speed at given RPM
0 km/h
Speed (mph)
0
Final drive ratio
0
Tire circumference
0 in
Tire revs / km
0
RPM @ 100 km/h
0
RPM @ 60 mph
0

Notes

  • Final drive ratio = transmission ratio × differential ratio. The full reduction from engine to wheel.
  • Lower numerical axle (e.g., 3.08) = taller gearing, lower cruise RPM, slower acceleration.
  • Higher numerical axle (e.g., 4.10) = shorter gearing, snappier acceleration, higher cruise RPM = less fuel-efficient.
  • To approximate tire diameter from a P-metric size (e.g. 225/65R17): diam = wheel + 2 × (width × aspect/100 / 25.4) inches.