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Gear Ratio Speed Calculator

Enter your engine RPM, gearing and tire diameter to find the resulting road speed.

How it is calculated

The tool works from the engine to the road in four steps. Total ratio is gear × final drive. Wheel speed is wheel rpm = engine rpm ÷ total ratio. Distance per wheel turn is the tyre circumference π × diameter, with the diameter in inches. Then mph = wheel rpm × circumference × 60 ÷ 63,360 (63,360 inches in a mile), and km/h is mph × 1.609344. It reports speed in mph and km/h, wheel rpm and the total ratio (three decimals). Both ratios and the diameter must be above zero; RPM may be 0. The tool assumes no wheel slip and no torque-converter slip, and uses the diameter you enter, which is why a loaded tyre, which is slightly smaller than its nominal size, gives a slightly lower real speed.

Worked example

An engine turns 3,000 rpm in a gear of 0.75 with a 3.73 final drive and a 26-inch tyre. The total ratio is 0.75 × 3.73 = 2.7975, shown as 2.798:1. Wheel speed is 3000 ÷ 2.7975 = 1,072.39 rpm. The circumference is π × 26 = 81.68 in, so the car covers 1,072.39 × 81.68 = 87,592 in per minute. Multiplying by 60 and dividing by 63,360 gives 82.95 mph, which is 133.49 km/h.

Try this example →

Common mistakes

  • Entering the tyre diameter in millimetres or centimetres, or the rim size instead of the overall tyre diameter; the field expects the full outside diameter in inches.
  • Typing the gear ratio inverted, for example 1.33 instead of 0.75 for an overdrive gear; overdrive ratios are below 1.
  • Entering the final drive as 373 instead of 3.73, which makes the speed 100 times too low.
  • Forgetting that two-speed transfer cases, or a separate low range, add another ratio that must be multiplied into the gear ratio or the final drive.
  • Expecting the speed to match the speedometer exactly; the tool ignores slip and the difference between nominal and loaded tyre size.

About this calculator

A vehicle's road speed at a given engine RPM depends on three multiplying factors: the selected gear ratio, the final drive (differential) ratio, and the tire diameter, which together determine how many times the wheels rotate per engine revolution and how much ground each wheel rotation covers. The formula is Speed = (RPM × tire circumference × 60) / (gear ratio × final drive ratio × 63360) for mph (with unit constants adjusted for km/h), since tire circumference converts wheel rotations directly into distance traveled.

Drag racers and circle track racers tuning gear selection, off-road and 4x4 builders choosing tire and axle gear combinations, and automotive engineers and enthusiasts use this calculation to predict top speed in each gear, check whether an engine will stay in its powerband at redline, or verify that a tire size change won't throw off the speedometer.

Engine RPM, the gear ratio, the final drive and the tire diameter go in, and road speed comes out in both mph and km/h, quicker than doing the ratio arithmetic by hand for each gear and rev point.

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Comments (1)

  • Editorial team

    The tyre box wants overall diameter, not rim size. A 245/40R18 works out to about 25.7 inches: two sidewalls of 98 mm plus an 18-inch rim. Put the engine speed you are interested in, such as the redline, to see the road speed that gear reaches, and use a ratio below 1 for an overdrive gear. Loaded tyres are slightly smaller, so expect a touch less on the road. Which tyre size do you run?

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