Speedometer Gear Calculator

What Is a Speedometer Gear?

In vehicles with a mechanical speedometer, speed is measured by gears inside the transmission.

There are two main parts:

• Drive gear: Mounted on the transmission output shaft
• Driven gear: Connects to the speedometer cable

As the driveshaft spins, the drive gear turns the driven gear. The driven gear then spins the speedometer cable, which moves the needle on your dash.

If tire size or axle ratio changes, the driveshaft speed changes too. The original gear setup no longer matches reality. That is where a speedometer gear calculator is needed.

What Does a Speedometer Gear Calculator Do?

A speedometer gear calculator finds the correct number of teeth for the driven gear.

It uses four main inputs:

• Drive gear tooth count
• Axle (differential) ratio
• Tire size
• Load factor (tire squat)

From these, it calculates:

• Tire diameter
• Revolutions per mile
• Exact driven gear tooth count
• Rounded gear size you should install

If you enter your current driven gear, it can also show speed error and real road speed.

Why Speedometer Accuracy Matters

An inaccurate speedometer causes real problems.

• You may drive faster than the speed limit without knowing it
• Fuel economy calculations become unreliable
• Odometer readings drift over time
• Transmission shift points may feel off

Even a small tire size change can create a noticeable error. A proper gear correction fixes all of this at the source.

Understanding the Calculator Inputs

This calculator is designed to be technical but practical. Here is what each input means.

Drive Gear Teeth

This is the number of teeth on the gear attached to the transmission output shaft.

Common values are 6, 7, or 8 teeth.
This gear usually requires transmission removal to change.

Axle Ratio

The axle ratio shows how many times the driveshaft turns for one wheel rotation.

Examples:

• 3.08 for highway-focused cars
• 3.73 or 4.10 for performance builds

Higher ratios increase driveshaft speed and affect speedometer accuracy.

Tire Size Explained (Width / Aspect / Rim)

The calculator uses standard tire sizing.

Example: 245 / 45 R17

• 245 = tire width in millimeters
• 45 = sidewall height as a percentage of width
• 17 = wheel diameter in inches

This data is used to calculate tire diameter and revolutions per mile. Those values directly affect speedometer calibration.

Load Deflection (Tire Squat)

Tires flatten slightly when the vehicle is on the ground. This reduces the effective rolling diameter.

The calculator gives two options:

• Standard (3% squat): Best for real-world driving
• Theoretical (no load): Best for math-only comparisons

For street vehicles, the standard option is usually more accurate.

How the Speedometer Gear Formula Works

Behind the scenes, the calculator follows a clear process.

Step 1: Tire Diameter

Tire diameter is calculated using width, aspect ratio, and rim size.

This converts metric values into inches.

Step 2: Revolutions Per Mile

The calculator finds how many times the tire turns in one mile.

It uses:

• Tire diameter
• Pi
• Load factor

Smaller tires = more revolutions per mile
Larger tires = fewer revolutions per mile

Step 3: Driven Gear Teeth Formula

The core formula is:

Driven Gear Teeth =
(Drive Gear Teeth × Axle Ratio × Revs Per Mile) ÷ 1000

The result is an exact number.
The calculator rounds it to the nearest whole tooth for real-world use.

Reading the Calculator Results

Once you click Calculate Teeth, the results section appears.

Recommended Driven Gear

This is the gear you should install.

It is shown as a whole number, which matches how gears are sold.

Supporting Data

The calculator also shows:

• Exact theoretical tooth count
• Tire diameter in inches
• Revolutions per mile
• Calibration base (1000 cable revs per mile)

This data helps you double-check the math if needed.

Speedometer Error Check (Optional but Useful)

If you enter your current driven gear, the calculator compares it to the correct value.

It then shows:

• Speed error percentage
• Real speed when the speedometer reads 60 mph

Example:
If the display says +6%, your speedometer reads faster than actual speed.

This feature is useful when diagnosing existing errors.

Expert Advice Logic Explained

The calculator also gives plain advice based on the result.

• Very large driven gear: Suggests changing the drive gear instead
• Very small driven gear: Warns about possible binding
• Normal range: Confirms compatibility and mentions common color coding

This helps prevent ordering parts that will not physically fit your transmission.

When You Should Use a Speedometer Gear Calculator

Use this tool if you have:

• Changed tire size
• Installed new axle gears
• Swapped transmissions
• Noticed speedometer error

It is especially useful for classic cars, trucks, and performance builds that still use mechanical speedometers.