Speedometer Gear Calculator

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.

Do you prefer bikes over cars? Then you can check out some of our bike-related tools, such as the bike gear calculator to determine the gear ratio for a bike 🚲, or the car vs bike calculator to find out the benefits of biking instead of driving.

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.

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:

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

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:

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.

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.