Speedometer Gear Calculator

By Rahul Dhari
Last updated: Aug 03, 2021

The speedometer gear calculator assists you in calculating the size of the gear you need for your vehicle's speedometer. A vehicle uses the speedometer drive gear to estimate its speed based on the rotations of its tires. Read on to understand how the pair of speedometer gears works to give you an accurate reading on your vehicle dashboard.

What is a speedometer gear?

A speedometer is a device that is used to measure the speed of a moving object, like a car or bike. Vehicles with such speedometers can display their instantaneous speed at any time. They were invented by a Croatian professor named Josip Belušić, in the year 1888. They can be either analog or digital in the display.

A mechanical speedometer houses a needle or pointer which is linked to the driven gear and is in turn attached to the vehicle's transmission output. The digital version of this measuring device consists of a rotation sensor attached to the transmission shaft. The car's computer uses the output of this sensor to convert the signals into speed and displays it on the vehicle dashboard.

Ford Mondeo MK3 ST220 - Speedometer
Speedometer Credit: Wikimedia CC BY 2.0

The focus of this calculator is on the mechanical speedometer, which houses a gear. As a result of knowing what a speedometer is, we can call the gear in the gauge the speedometer drive gear. We often change tires and sometimes switch to bigger or smaller tires. As we change the tires, the speedometer gear also needs to be adjusted to keep an accurate reading.

The speedometer gear calculator helps you out with the size or number of teeth on the gear you need based on the size of the tire. Say, for example, you need to calculate the gear ratio for bigger tires. Mathematically, you'll have to first calculate the estimated number of tire revolutions, R per unit distance (miles or kilometers), like this:

R = 20168 / D,

where D is the diameter of the tire in inches, and the numerator 20168 can also be written as 63360/π such that 63360 inches = 1 mile. We then go on to use the revolution parameter, R to estimate the gear ratio, G for bigger tires.

G = A * R / 1001,

where A is axle ratio, 1001 denotes that the speedometer is calibrated to 1 mile for 1000 revolutions, and the gear ratio G can be written as:

G = No. of teeth on driven gear / No. of teeth on drive gear

Using the speedometer gear calculator

Follow the steps below to estimate the suitable speedometer gear.

  1. Enter the diameter of the tire, D.
  2. The calculator will now return the number of revolutions per mile.
  3. Enter the axle ratio, A.
  4. Insert the value of the number of teeth on the drive gear.
  5. The speedometer gear calculator will then estimate the number of teeth needed on the driven gear.

Note: The output of the calculator is rounded off to the next integer. You might also need to round off to the next value of the gear size available, unless you want to 3D print it.

Example: Using the speedometer gear calculator.

Determine the size of drive gear for the speedometer given the tire size is 20 inches and the axle ratio is 3. Assume the number of teeth on driving gear as 40.

To solve the above problem:

  1. Enter the diameter of the tire, D = 20 inches.

  2. The calculator will now return the number of revolutions per mile, R = 1008.4.

  3. Enter the Axle ratio, A = 3.

  4. Insert the value of the number of teeth on the driving gear, as 40.

  5. Using the two equations from above, we get:

    No. of teeth on driven gear = A * R * No. of teeth on drive gear / 1001 = 121

    Therefore, you would need a drive gear with 121 teeth for the tire size of 20 inches.

FAQ

What is a speedometer?

The speedometer is a device that measures the speed of a moving vehicle and displays the output on the dashboard meant for the driver.

What is gear ratio?

The gear ratio is defined as the ratio of the number of rotations of the driving gear to the number of rotations of the driven gear.

What are the components inside a speedometer?

A speedometer consists of a drive gear that is liked to the vehicle transmission output on one end and to the needle on the dashboard on the other end. The rotation of the gears helps in determining the speed based on the number of teeth it has.

How to calculate the number of teeth on speedometer drive gear?

To calculate the number of teeth on speedometer drive gear:

  1. Divide the number 20168 by the diameter of the tire to find the number of revolutions per mile.
  2. Multiply the revolutions with axle ratio, A.
  3. Divide the product with 1001.
  4. Multiply the gear ratio with the number of teeth on the driven gear, N. The gear ratio, G, is given by:
G = A * R * N / 1001
Rahul Dhari
Diameter of tire (D)
in
Axle ratio (A)
Revolutions
per
mi
Number of teeth on driving gear
Number of teeth on driven gear
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