Vehicle Gear Ratio (Final Drive) Calculator
RPM = (MPH × 336 × ratio) / tire-diameter. Find RPM at given speed for any axle ratio.
Result
Engine RPM
1,901 rpm
65 mph in top gear (0.7 × 3.73 = 2.611).
- Speed65 mph
- Axle ratio3.73:1
- Top gear ratio0.7:1
- Effective ratio2.611:1
- Tire diameter30 in (94.2 in circ)
- Engine RPM1,901
- Sweet spot?✓ Cruise
Step-by-step
- Effective ratio = top gear × axle = 0.7 × 3.73 = 2.611.
- RPM = (MPH × ratio × 336) / tire diameter = (65 × 2.611 × 336) / 30 = 1,901.
How to use this calculator
- Enter speed and your axle ratio.
- Estimate top gear (auto: 0.7; manual 5sp: 0.85).
- Read RPM.
About this calculator
Vehicle final-drive ratio = top gear × axle ratio. Lower numerical ratio (e.g. 3.21:1) = lower cruise RPM = better highway MPG. Higher ratio (4.10:1+) = quicker acceleration, towing, but higher cruise RPM and worse MPG. Lifted trucks with bigger tires often re-gear to compensate. Sweet-spot cruise RPM: 1800-2500. Above 3000 at 65 mph = poor highway MPG. The 336 in the formula = (60 min × 1056 ft/min @ 1mph × 12 in/ft) / (π × 1 in tire dia) approximation.
Frequently asked
Owner's manual, door jamb sticker, dealer (RPO codes for GM), or count diff teeth.
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