Vehicle Gear Ratio (Final Drive) Calculator

RPM = (MPH × 336 × ratio) / tire-diameter. Find RPM at given speed for any axle ratio.

Inputs

Modern auto: 0.6-0.8 in top gear. Direct drive: 1.0.

Result

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General calculation reads

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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

How do I find my axle ratio?+
Owner's manual, door jamb sticker, dealer (RPO codes for GM), or count diff teeth.
Common axle ratios?+
Trucks: 3.42, 3.55, 3.73, 4.10. SUVs: 3.21-3.73. Sports cars: 3.36-4.30. EV: single-speed gearbox 7-9:1.
Why re-gear with bigger tires?+
Bigger tires effectively lower the final ratio (slower RPM at speed), hurting acceleration. Re-gearing higher restores lost performance.
CVT?+
No fixed top gear ratio — CVTs vary continuously. Ignore this calc; CVT cruise RPM is whatever the controller picks (usually 1500-2000 at 65 mph).
EV equivalent?+
EVs have a single fixed gear (7-9:1). RPM at speed = MPH × gear × 336 / tire diameter. EV motors red-line ~12,000-15,000 RPM.

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