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Gear Ratio Calculator

Enter the tooth counts of two meshing gears to get the gear ratio. Add a final drive ratio, engine RPM and tire diameter and the calculator also gives the overall ratio and the road speed that gearing produces.

Quick examples

The gear that supplies the power (input or pinion).

The gear being turned (output or ring gear).

Axle or differential ratio. Enter 1 if the gear pair above is the final drive.

RPM
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Gear ratio1.5 : 1
Overall ratio with final drive
5.595 : 1
Wheel speed
536.2 RPM
Road speed
41.47 mph

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      Formula

      Gear ratio = driven teeth ÷ driving teeth
      Overall ratio = gear ratio × final drive ratio
      Wheel RPM = engine RPM ÷ overall ratio
      Speed (mph) = wheel RPM × tire diameter (in) × π × 60 ÷ 63,360

      How to use it

      1. Enter the number of teeth on the driving (input) gear.
      2. Enter the number of teeth on the driven (output) gear.
      3. Enter the final drive ratio, or 1 if there is none.
      4. Enter engine RPM and tire diameter to see road speed.

      Worked examples

      A 24-tooth gear driving a 36-tooth gear, a 3.73 final drive, 3,000 RPM and 26 in tires

      Gear ratio
      1.5 : 1
      Overall ratio with final drive
      5.595 : 1
      Wheel speed
      536.2 RPM
      Road speed
      41.47 mph

      A 1:1 top gear (30 and 30 teeth) with a 3.42 axle at 2,500 RPM on 27 in tires

      Gear ratio
      1 : 1
      Overall ratio with final drive
      3.42 : 1
      Wheel speed
      731 RPM
      Road speed
      58.72 mph

      What the ratio tells you

      A ratio above 1, such as 3.73:1, is a reduction: the input turns 3.73 times for each turn of the output, multiplying torque by the same factor and cutting speed. A ratio below 1 is an overdrive, trading torque for speed.

      In a vehicle, the transmission gear ratio and the axle (final drive) ratio multiply. A 0.70 overdrive top gear with a 3.73 axle gives 2.61 overall. For a ring and pinion, the ring gear is the driven gear: 41 ring teeth and 11 pinion teeth is 3.73:1.

      Speed at a given RPM

      A shortcut for road speed is mph = RPM × tire diameter in inches ÷ (overall ratio × 336). The calculation here ignores tire growth, tire deflection under load and torque converter slip, so a real vehicle will be a little slower than the figure shown.

      Numerically higher axle ratios (4.10 rather than 3.42) give stronger acceleration and towing at the price of higher engine speed and fuel use when cruising. Fitting taller tires has the opposite effect.

      Questions people ask

      How do I calculate a gear ratio?

      Divide the teeth on the driven gear by the teeth on the driving gear. A 36-tooth gear driven by a 24-tooth gear is 36 ÷ 24 = 1.5:1.

      What is a 3.73 gear ratio?

      The driveshaft turns 3.73 times for every turn of the wheels. A ring gear with 41 teeth and a pinion with 11 teeth gives this ratio.

      How fast am I going at 3,000 RPM?

      It depends on gearing and tire size. In a 1:1 gear with a 3.73 axle and 26 in tires, 3,000 RPM is about 62 mph.

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      Tire Size CalculatorTire diameter and revs per mile, plus speedometer error vs a new size.Horsepower CalculatorHorsepower from torque and RPM, or from quarter-mile results.Ratio CalculatorSimplify a ratio and split a total in that ratio.Speed CalculatorSolve speed, distance or time from the other two.Circumference CalculatorDistance around a circle from its radius, diameter or area.