Speedometer Gear Calculator

Calculate the correct transmission speedometer driven gear tooth count. Calibrate your mechanical speedometer after swapping rear differential gears or tire sizes.

Speedometer Gear Calculator

Input Custom Tire Diameter Directly

Drive Gear Tooth Count

teeth

Rear Axle Gear Ratio

Tire Specifications

Section Width

Aspect Ratio

Wheel Size

Please check your inputs. All mechanical parameters must be positive numbers.

Required Driven Gear Size

20 Teeth

Exact Calculated Tooth Count: 0.00 teeth

Tire Diameter: 0.00 inches

Tire Revolutions Per Mile: 0.0 revs/mi

The Mechanical Speedometer Calibration Formulas

On older and classic vehicles (including vintage muscle cars, Jeeps, and classic trucks), speedometers are driven mechanically via a flexible cable linked to the transmission tailshaft. If you change your rear differential gear ratio (ring and pinion set) or your rear tire size, the speedometer calibration is altered.

To correct this, you must change the plastic speedometer driven gear on the cable end. The mechanical equations used to calculate the correct number of gear teeth are outlined below:

Step 1: Calculate Tire Revolutions Per Mile

Determine the total tire diameter and how many revolutions the tire performs over the distance of one mile. The mathematical constant 20,168 is used to translate diameter to revolutions:

Tire Diameter (inches) = [Width (mm) × (Aspect Ratio % ÷ 100) × 2 ÷ 25.4] + Wheel Size (inches)Tire Revolutions Per Mile = 20,168 ÷ Tire Diameter (inches)

Step 2: Calculate Driven Gear Tooth Count

Using the tire revolutions, your transmission's internal drive gear tooth count, and your rear axle gear ratio, apply the standard mechanical calibration formula:

Driven Gear Teeth = (Drive Gear Teeth × Axle Ratio × Tire Revs Per Mile) ÷ 1,001

Note: Because gear teeth are distinct physical entities, round the exact decimal result to the nearest whole integer to select your replacement driven gear.

Driven Gear Quick Reference Table

The table below showcases the calculated driven gear requirement across standard rear differential gear ratios.

Baseline Assumptions: This calculation utilizes a standard 7-tooth transmission drive gear (common on many Ford T-5 and GM transmissions) and a standard 26-inch tall tire (equivalent to approximately 775.7 revolutions per mile).

Rear Axle Gear Ratio	Exact Calculated Driven Teeth	Recommended Driven Gear (Rounded)
2.73 Ratio	14.81 teeth	15-Tooth Gear
3.08 Ratio	16.71 teeth	17-Tooth Gear
3.42 Ratio	18.55 teeth	19-Tooth Gear
3.73 Ratio	20.23 teeth	20-Tooth Gear
4.10 Ratio	22.24 teeth	22-Tooth Gear
4.56 Ratio	24.73 teeth	25-Tooth Gear

How to Verify Your Drivetrain Specifications

To run this calibration calculation, you need to identify three variables from your vehicle's build sheet or mechanical assembly:

1. Drive Gear Teeth vs. Driven Gear Teeth

It is important not to confuse these two distinct components:

Drive Gear (Internal): This gear is mounted directly onto the transmission's rotating output shaft inside the tailhousing. It is often a steel or hard plastic gear. To change this gear, you typically have to remove the tailhousing or disassemble parts of the transmission. Most common classic transmissions have drive gears with 6, 7, 8, or 9 teeth.
Driven Gear (External): This is a small, inexpensive, colored plastic or nylon gear mounted to the end of the removable speedometer cable housing. To change this gear, you simply unscrew the retaining clip on the side of the transmission tailhousing, pull the cable out, and swap the plastic clip-on gear.

2. Determining Your Axle Ratio

If you do not know your rear differential axle ratio, you can find it using these methods:

Axle Tag: Look for a small metal tag bolted to the differential cover. It is often stamped with the ratio (e.g., "3 73" or "3L73" for a 3.73 limited-slip axle).
Manual Rotation Method: Safely raise the rear wheels of the car off the ground (utilizing jack stands and wheel chocks). Place the transmission in neutral. Rotate the driveshaft by hand and count how many full revolutions it takes to rotate the rear tires exactly one full turn. If the driveshaft turns roughly 3 and 3/4 times to turn the tire once, your axle ratio is 3.73.

3. Choosing the Closest Wrench/Gear Part

Plastic driven gears are sold in distinct teeth sizes (usually from 16 to 23 teeth depending on the manufacturer). If your calculated value is a decimal like 19.4, select a 19-tooth gear. If your value is 19.6, select a 20-tooth gear. Selecting the closest matching integer ensures your speedometer reading is accurate to within a few percentage points of your actual ground speed.

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