How to calculate speed based on gear ratios

Written by robert bayly
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How to calculate speed based on gear ratios
(Michael Hitoshi/Photodisc/Getty Images)

Gear ratio can have a profound effect on how your vehicle performs. Moving to a higher (numerically lower) ratio can improve your freeway mileage because your engine is turning fewer RPMs, but the trade-off is that the vehicle will not accelerate as quickly because fewer engine revolutions are being applied to the rear wheels. The flip side is that a lower (numerically higher) ratio will improve your acceleration at the expense of mileage because the engine is turning faster for any given speed. As with all things mechanical and measurable, you can calculate these variables with a formula.

Skill level:
Moderately Easy

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Things you need

  • Measuring tape
  • Calculator

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Instructions

  1. 1

    Park your vehicle on a level, firm surface and set the parking brake.

  2. 2

    Use a measuring tape to determine how tall your tire is. This is the outside diameter of the tire. Write this figure down.

  3. 3

    Write down the gear ratio you want to calculate for, for instance, "3.73." Determine the final drive ratio. This is the output ratio of the transmission multiplied by the gear ratio. For instance, a normal four-speed transmission has an output ratio of "1-to-1" (written 1:1) in fourth gear. This means that for every one engine revolution, the output shaft on the transmission turns once. A common five-speed final ratio is .80:1. This means the engine is turning eight-tenths of a revolution for one turn of the transmission. This is called an "overdrive" ratio. Using 3.73 as our example, you would have "3.73 X 1 = 3.73" for the four-speed and "3.73 X .8 = 2.98" for the five-speed.

  4. 4

    Write down the engine RPM you want to calculate the MPH value for. For instance, you want to know how fast you will be going at 2,000 RPM with a certain gear ratio. You will enter this number in the formula in the next step. You can choose any RPM value that you want, but 2,000 RPM is used in the example below. You will be able to calculate the MPH for any RPM and gear ratio by changing those values.

  5. 5

    Enter the above figures into a calculator in the following manner: (RPM x Tire Diameter)/(Final Drive Ratio x 336) = MPH. The number "336" is the "constant" and is part of the formula. Here is an example of the formula for a four-speed transmission at 2,000 RPM, with 26-inch tires and a final drive of 3.73: (2,000 x 26)/(3.73 x 336) = 52,000 divided by 1,253.28 = 41.49 MPH. With a .8 overdrive five-speed, the answer would be 51.93 MPH.

Tips and warnings

  • Here are the formulas for related calculations:
  • Gear Ratio = (RPM x Tire Diameter)/(MPH x 336)
  • RPM = (MPH x Gear Ratio x 336)/Tire Diameter
  • Tire Diameter = (MPH x Gear Ratio x 336)/RPM

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