Speedometer Gear Calculator helps you find the size of the gear you need for your vehicle’s speedometer. Check the number of teeth on the driven gear needed for your vehicle.

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Speedometer – Definition

A speedometer is a device designed to measure the current speed of a moving vehicle, a car, bike, or any other vehicle. Going back to 1888, it was the important year for vehicles when the Croatian Josip Belušić designed and invented the first speedometer. It was an analog speedometer back then, but today digital speedometers took over the market.

Illustration of a speedometer gear

How does it work in practice?

The digital speedometer uses the rotation sensor attached to the transmission shaft of the vehicle. As a result, the car’s computer uses the rotation sensor, reads its transmission output, and converts the signals into speed by displaying the information on the vehicle dashboard.

Speedometer Gear Formula

Whenever you want to calculate the gears ratio or the number of teeth on the gear, you need to go through several formulas.

The first step is to find the number of tire revolutions per unit of distance, with the following formula:

R = \frac{20168}{D}

D – the diameter of a tire

Once the tire revolutions parameter is known, we can proceed with the calculation by finding the gear ratio (G). The gear ratio for bigger tires is calculated by the equation:

G = \frac{A \times R}{1001}

A – Axle ratio (number of revolutions the transmission output shaft needs to make to spin the axle for one complete turn)
R – Tire revolutions

The final formula that is used to find the gears ratio uses the number of teeth of driving and on driven gear:

G =  \frac{\text{Number of teeth on the driven gear}}{\text{Number of teeth on the driving gear}}

How Do I Know Which Speedometer Gear Do I Need?

What steps are you required to take to determine what speedometer transmission gear will fit your vehicle? Let’s find out. Note: This method can be used for calculating the speedometer gear for any vehicle: Mustang, Ford etc.


  • Start with counting how many teeth are on the speedometer transmission drive gear.
  • Calculate the gears ratio for the drive axle
  • Use the formula from above and calculate tire revolutions per unit of distance.
  • Determine how many teeth are required for the driven gear
  • Based on your calculation, purchase the driven gear for your vehicle with the closest tooth count compared to the results of your calculation.

For example, Ford speedometer transmission gear comes in some common types (Drive gears – Driven Gears):

  • 6 teeth – 16 teeth
  • 7 teeth – 19 teeth
  • 8 teeth – 20 teeth
  • 9 teeth – 21 teeth

Speedometer Gear Calculator – How to Use?

Do not be frightened by all these crazy formulas and the things you never heard of before. With our easy-to-use calculator, you can find the number of teeth needed on the driven gear in a few clicks. Don’t you believe it? Let me take you through the steps.

Step-by-step process:

  1. Figure out what’s the diameter of your tire (D).
  2. Immediately, our calculator will apply the formula and calculate the tire revolutions per mile or kilometers (R).
  3. Enter the axle ratio (A) and the number of teeth on the driving gear.
  4. The calculator will return the number of required teeth on the driven gear.

As simple as that – Don’t bother with the formulas!

Speedometer Gear Calculator – Example

Let’s go one step further and use Speedometer Gear Calculator in a real example below to show how it performs calculations.

Scenario: Suppose your vehicle’s tire has a diameter of 18 inches, the axle ratio is 4, and it has 35 teeth on the driving gear. How would you find the number of teeth on the driven gear using Speedometer Gear Calculator?

  • Enter the diameter of a tire in the calculator: 18 inches.
  • The calculator uses the diameter size and calculates revolutions per mile: R = 1120.4 per mile.
  • Enter the axle ratio (A) and the number of teeth on the driving gear.
  • As a result, the calculator will estimate the number of teeth required on the driven gear – 157.

You can easily follow the same process for any other scenario. Simply change the values to match yours and find the required number of teeth on the driven gear for your vehicle.


What is the gear ratio?

The gears ratio is a ratio between the required teeth on the driving gear and the required teeth on the driven gear.

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

You can calculate it using the following steps:
– Find (R) Revolutions per mile by dividing (20168) by the tire’s diameter (inches)
– Take the number of revolutions per mile and multiply it by A
– Divide the result from the previous calculation by 1001
– Multiply the given product by the number of teeth on the driven gear

How do I slow down my speedometer?

You can slow down speedometer readings by changing your vehicle’s tire height because most vehicles measure speed based on tire revolutions (R).

Which gear is the drive gear?

The drive gear, also known as a gearbox, is normally connected to the motor or engine of a vehicle. Its function is to:
– Increase torque from the motor to the driven equipment.
– To reduce the speed given by the motor.
– And to change the direction of the rotating shaft.

Does putting bigger wheels on your car affect the speedometer?

Putting bigger wheels on your car will make the axle rotate fewer times for the same given distance. Thus, the speedometer will think and display that your car is moving slower.

How much does it cost to recalibrate the speedometer?

It depends on the country where you live. However, roughly it would cost you $100.

How accurate is my car speedometer?

If you never change the diameter size of your tires, the speedometer will have solid accuracy. On the contrary, if you put smaller or bigger wheels on your car (even a slight difference in millimeters), it will make the speedometer display an incorrect estimated moving speed.

Is GPS or speedometer more accurate?

GPS measuring is more accurate than the speedometer. However, for GPS to give you the correct speed, you need to have a stable signal to the satellites, preferably under the clear sky.