WheelSignal

WheelSignal

Results printed from https://wheelsignal.com/gear-ratio-calculator. Nominal estimates; see assumptions below.

Gear ratio calculator

Find the theoretical road speed at a given engine RPM, or the RPM at a given speed, for each gear you enter.

Gearing, RPM and road speed

Tire rolling size
Uses the nominal circumference from the size.
Ratios
e.g. 1.00 for a 1.00:1 gear
Axle / differential ratio
Separate with commas, lowest gear first. Up to 12 ratios.
Calculate
Unit: RPM
Unit: RPM Display range only, not an engine limit

Unit choices are saved only if you tick the box, in this browser’s local storage. Measurements are never saved or sent anywhere.

Results

Enter tire size, ratios and an RPM or speed, or press Try an example, then Calculate.

How to use the gear ratio calculator

  1. Describe the driven tire: by size (for example 205/55 R16), by rolling circumference, or by the manufacturer’s revolutions per mile or kilometre.
  2. Enter the gear ratio you want to look at and the final-drive ratio. Add the full list of gear ratios to compare every gear.
  3. Choose whether to find road speed at an engine RPM, or engine RPM at a road speed, and enter that value.
  4. Press Calculate for the result, a table across all gears and a speed-versus-RPM chart.

Ratio definitions and units

Ratios are written as input turns per output turn. A gear of 3.50 (3.50:1) turns the engine 3.5 times for each gearbox output turn; an overdrive gear below 1.00, such as 0.80, turns the output faster than the engine. The final-drive ratio is the reduction in the differential or axle. The overall ratio is gear × final drive: engine revolutions per wheel revolution.

Speeds are in mph or km/h, circumference in millimetres or inches, and engine speed in revolutions per minute. Everything is converted to metric internally using exact factors (1 in = 25.4 mm, 1 mile = 1.609344 km).

Worked example

Illustrative ratios, not a specific vehicle: a 205/55 R16 tire (nominal circumference 1,985 mm), a 1.00 fourth gear and a 4.00 final drive.

  • Overall ratio: 1.00 × 4.00 = 4.00, so at 3,000 RPM the wheels turn 3,000 ÷ 4 = 750 times a minute.
  • Road speed: 750 × 1.9852 m × 60 ÷ 1,000 = 89.3 km/h (55.5 mph).
  • In a 0.80 fifth gear at 100 km/h, the engine would turn at about 2,687 RPM.
Theoretical road speed versus engine RPMGear 1 (3.50): 8.5 km/h per 1000 RPM; Gear 2 (2.10): 14.2 km/h per 1000 RPM; Gear 3 (1.40): 21.3 km/h per 1000 RPM; Gear 4 (1.00): 29.8 km/h per 1000 RPM; Gear 5 (0.80): 37.2 km/h per 1000 RPM05010015020025001,0002,0003,0004,0005,0006,000Engine speed (RPM)Road speed (km/h)Gear 1 (3.50)Gear 2 (2.10)Gear 3 (1.40)Gear 4 (1.00) ◆Gear 5 (0.80)89.3 km/h @ 3,000 RPMTheoretical, assumes no slip. Axis range is not an engine limit.

Formulas used

overall ratio = gear ratio × final-drive ratio wheel rev/min = engine RPM ÷ overall ratio speed (km/h) = wheel rev/min × circumference (m) × 60 ÷ 1,000 speed (mph) = speed (km/h) ÷ 1.609344 engine RPM = speed (km/h) × 1,000 ÷ 60 ÷ circumference (m) × overall ratio circumference (m) = 1,609.344 ÷ revolutions per mile

Limitations

  • Assumes a direct mechanical connection with no clutch, torque-converter or tire slip.
  • Variable-ratio systems (CVTs, power-split hybrids) and unknown transfer-case or drop-gear ratios are not supported unless you include them in the ratios.
  • Nominal tire circumference is usually a few percent larger than the loaded rolling circumference.
  • Does not predict top speed, acceleration, engine suitability or safe engine speed, and does not contain any vehicle specifications.

Frequently asked questions

What do gear ratio and final-drive ratio mean?

A gear ratio of 3.50 (often written 3.50:1) means the engine turns 3.5 times for each turn of the gearbox output. The final-drive (axle or differential) ratio does the same between the gearbox output and the wheels. Multiplying them gives the overall ratio: engine revolutions per wheel revolution.

Why is my real RPM different from the calculated figure?

The calculation assumes no slip anywhere. Torque converters in automatic gearboxes slip unless locked, clutches slip while engaging, and tyres creep slightly under drive. The loaded rolling circumference of a tyre is also a little smaller than the nominal figure from its size, and speedometers often read slightly high.

Does this work for CVTs, hybrids or electric vehicles?

Only where the ratio is fixed and known. Continuously variable transmissions (CVTs) and power-split hybrid systems change ratio continuously, so a single gear ratio does not describe them. Many electric vehicles use a single fixed reduction, which you can enter as the gear ratio with a final drive of 1.00 if the published figure is the total reduction.

Can it tell me my top speed or the safe maximum RPM?

No. Top speed depends on power, drag, gearing and limiters, and safe engine speed depends on the engine. The chart’s RPM axis is only a display range that you set; it is not an engine limit. Check the vehicle manufacturer’s information.

Should I use tyre size or revolutions per mile?

If the tyre manufacturer publishes revolutions per mile or per kilometre for your exact tyre, use it: it reflects the loaded rolling circumference. A tyre size gives the nominal circumference, which is usually a few percent larger than the rolling value.

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