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Calculate engine horsepower from torque and RPM or estimate from quarter-mile time and weight. Convert between HP, kW, and brake horsepower units.
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Engine RPM equals vehicle speed times total gear ratio times a constant divided by tire diameter. The constant 336 applies when speed is in mph and tire diameter in inches.
RPM = (Speed × Gear ratio × Final drive × 336) / Tire diameterMaximum speed in a gear occurs at redline RPM. Rearrange the RPM formula to find speed at any RPM and gear combination.
Speed = (RPM × Tire diameter) / (Gear ratio × Final drive × 336)Lower numerical final drive (e.g., 3.08 vs 3.73) reduces highway RPM and improves fuel economy. Higher numerical ratio improves acceleration but increases RPM at cruise.
Updated: July 2026
6th gear 0.85, final drive 3.55, 26-inch tires: RPM = (70 × 0.85 × 3.55 × 336) / 26 = 2,430 RPM. Comfortable highway cruise.
Changing from 3.27 to 3.90 final drive raises RPM at all speeds by 19%. 0-60 improves; highway RPM at 70 mph jumps from 2,100 to 2,500.
Larger 28-inch tires (vs 26-inch) reduce effective gearing by 7%. Same RPM at speed as a lower numerical final drive — recalculate after tire changes.
Lower gearing means a higher numerical ratio (3.90 is lower gearing than 3.08). Lower numerical ratio = taller gearing = less acceleration, lower highway RPM.
Total gear ratio is transmission gear times final drive (and transfer case if 4WD). Omitting final drive produces RPM values that are off by 3–4×.
Gear ratios and final drive determine engine RPM at highway speed, acceleration in each gear, and fuel economy. This calculator computes RPM at any speed, speed in each gear, and helps select final drive ratios for your tire size and performance goals.