Gear Ratio Speed Calculator: Speed from RPM, Gear, Axle Ratio and Tire Size
Calculate vehicle speed from engine RPM, transmission gear ratio, final drive (axle) ratio and tire diameter or tire size code, or find the engine RPM at a given speed, with a speed-per-gear table and gear chart up to redline.
How it works
mph = RPM × tire diameter (in) ÷ (gear ratio × final drive × 336.13) · RPM = mph × gear ratio × final drive × 336.13 ÷ tire diameter (in) · km/h = RPM × π × tire diameter (mm) × 60 ÷ (1,000,000 × gear × final drive) · tire diameter (in) = rim + 2 × width (mm) × aspect % ÷ 25.4
Engine RPM divided by the overall ratio (transmission gear × final drive) gives how fast the wheels turn. Each wheel turn moves the car one tire circumference, π × diameter, so wheel RPM × circumference × 60 is the distance per hour. In US units the constants fold into one number: 63,360 inches per mile ÷ (60 × π) = 336.13, which is the “336” used in axle-ratio formulas. Turn the formula around to find the RPM at a given speed, for example your highway cruise RPM. You can enter the tire as an overall diameter or as a sidewall size code: for 225/45R17 the sidewall is 225 × 45% = 101 mm, so the diameter is 17 in + 2 × 101 mm = 24.97 in. Add all your gear ratios under More options to get a speed-per-gear table and a gear chart up to your redline. Results assume no tire slip; a loaded tire is a little shorter than its listed size, so the real speed is slightly lower and the speedometer of a car with taller tires than stock reads low.
Worked example
At 3,000 rpm in a 1.00:1 gear with a 3.50 final drive and 26 in tires, the overall ratio is 1.00 × 3.50 = 3.50, so the wheels turn 3,000 ÷ 3.50 = 857.1 rpm. Speed = 3,000 × 26 ÷ (3.50 × 336.13) = 66.30 mph (106.7 km/h). The tire turns 776 times per mile, the engine turns 2,715 rpm at 60 mph, and at a 6,000 rpm redline this gear tops out at 133 mph.
Sources: PowerNation: Gear ratio calculator (336 constant, 33 in / 0.70 / 4.10 example) · Quadratec: Tire and gear ratio calculations (MPH, RPM, ratio formulas) · Omni Calculator: Transmission (speed from RPM, gear, differential and tire)
Frequently asked questions
What is the final drive ratio?
The ratio of the differential (ring and pinion) between the driveshaft and the wheels, such as 3.73:1. It multiplies with the transmission gear ratio to give the overall ratio. On a front-wheel-drive car it is built into the transaxle.
Where does the number 336 come from?
It converts RPM and tire inches into miles per hour: 63,360 inches in a mile divided by 60 minutes and by π gives 336.13. So mph = RPM × tire diameter ÷ (overall ratio × 336).
How do bigger tires change my speed and RPM?
A taller tire travels farther per turn, so the same RPM gives more speed and the engine turns fewer RPM at a given speed. It is like fitting a numerically lower axle ratio: acceleration drops and the speedometer reads low. To restore the original effective ratio, multiply the axle ratio by new tire diameter ÷ old tire diameter.
How do I find my tire diameter from the size code?
Multiply the width by the aspect ratio to get the sidewall height, double it and add the rim. For 205/55R16: 205 × 0.55 = 112.75 mm per sidewall, × 2 = 225.5 mm = 8.88 in, + 16 in = 24.88 in (632 mm). Choose “Size code” and the calculator does this for you.
What RPM will I cruise at on the highway?
Choose “RPM at a speed”, enter 60 or 70 mph and your top gear ratio. For example, 33 in tires, a 0.70 overdrive and 4.10 gears give about 2,046 rpm at 70 mph.
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This calculator is for educational purposes. Double-check important results before acting on them.