Speedometer Gear Calculator

Why Your Speedometer Might Be Lying to You

Have you ever changed the tire size on your vehicle or swapped out the rear axle gears for better performance? If so, there's a good chance your speedometer is no longer accurate. In vehicles with mechanical speedometers, the reading on your dash is controlled by a set of physical gears in the transmission. The ratio of these gears is precisely matched to your vehicle's original tire diameter and axle ratio. When you change either of those variables, you alter the speed at which the speedometer cable spins, leading to an incorrect speed reading. A Speedometer Gear Calculator is the essential tool for fixing this problem.

By inputting your new setup's details, the calculator tells you exactly which "driven gear" you need to install in your transmission to make your speedometer read correctly again. This is not just about avoiding speeding tickets; an accurate speedometer is crucial for correct odometer readings, which affects maintenance schedules and vehicle resale value.

How to Use the Speedometer Gear Calculator

To find the correct speedometer gear, you'll need to know three key details about your vehicle's drivetrain.

1. Drive Gear Teeth: The "drive gear" is located on the output shaft of your transmission. You will need to know the number of teeth on this gear. This often requires looking up your specific transmission model or, in some cases, a visual inspection. Common values are between 6 and 9 teeth.
2. Axle Gear Ratio: This is the gear ratio of your rear differential (e.g., 3.73, 4.10, 3.08). This might be on a tag on the differential housing or on the vehicle's build sheet.
3. Tire Diameter (inches): This is the overall height of your tire from top to bottom. You can measure this directly or use an online tire size calculator if you know your tire's size code (e.g., 275/60R15).

Once these values are entered, the calculator will output the required number of teeth for the "driven gear." Since gears only come in whole-tooth counts, you should choose the commercially available gear that is closest to the calculated result.

The Formula for Speedometer Accuracy

The calculator uses a well-established formula to determine the correct driven gear.

Step 1: Calculate Tire Revolutions per Mile

First, we need to know how many times your tire rotates to cover one mile.

Tire Revs per Mile = 20168 / Tire Diameter (in)

The constant 20168 is derived from dividing the number of inches in a mile (63,360) by Pi (π).

Step 2: Calculate Driven Gear Teeth

Next, the main formula combines all the factors.

Driven Teeth = (Drive Teeth × Axle Ratio × Tire Revs per Mile) / 1000

The formula divides by 1000 because this is the standard number of cable revolutions per mile that most American mechanical speedometers are calibrated for. The final result is the ideal number of teeth for the driven gear; you will need to select the closest available gear for purchase.

Practical Example

Imagine you've installed larger tires and a new axle gear set on your classic Ford Mustang.

• Your transmission's Drive Gear has 7 teeth.
• You've installed a new performance Axle Ratio of 3.90.
• Your new, taller tires have a Diameter of 27 inches.

Step 1: Find Tire Revolutions per Mile

20168 / 27 inches = 746.96 Tire Revs per Mile

Step 2: Calculate Required Driven Gear Teeth

Driven Teeth = (7 × 3.90 × 746.96) / 1000

Driven Teeth = 20383.1 / 1000 = 20.38

The calculator shows you need a driven gear with 20.38 teeth. Since you can't buy a gear with a fraction of a tooth, you would look for a 20-tooth driven gear, which is the closest available option.

Frequently Asked Questions (FAQ)

What if the calculated number is between two available gears?

Choose the closest whole number. If the result is 20.6, a 21-tooth gear will make the speedometer read slightly slower than your actual speed, while a 20-tooth gear will make it read slightly faster. Many people prefer the gear that results in a slightly faster reading to help avoid speeding.

Does this work for electronic speedometers?

No. This calculator is for older, mechanically driven speedometers that use physical gears. Modern electronic speedometers need to be recalibrated using a handheld electronic tuner that adjusts the signal from the vehicle's speed sensor.

Where do I buy speedometer gears?

Speedometer gears are specific to the make and model of your transmission. You can purchase them from specialty transmission shops, performance auto parts retailers, or dealership parts counters.

For more in-depth technical information on transmissions and gearing, please visit these expert sources:

• Modern Driveline - Speedometer Gear Info
• MotorTrend - How to Select Proper Speedometer Gears

To understand how these changes affect your engine speed, try our RPM Calculator.