Speedometer, Gear Ratio, and Tire Size Calculator

This page explains the formulas used to calculate tire circumference and vehicle speed from engine RPM, gear ratios, and tire dimensions. These calculations are useful for estimating speedometer error when fitting non-stock tire sizes, or for determining theoretical maximum speed in any gear.

The companion page TRANS-02 contains the full manual calculation formulas with worked examples. This page summarizes how to read a tire specification and how the underlying formulas work.

Reading a Tire Specification

A tire specification like 225/50VR16 breaks down as follows:

• Tread Width: 225 mm — the width of the tire contact patch
• Aspect Ratio: 50 — the sidewall height expressed as a percentage of tread width (50% of 225 mm = 112.5 mm sidewall)
• Wheel Diameter: 16 in. — the diameter of the rim in inches

How to Use These Formulas

Two formulas are needed: one to calculate tire circumference, and one to convert engine RPM into road speed.

Tire Circumference

The total tire diameter is built up from the wheel diameter plus two sidewall heights (one on each side). Because the tire specification mixes metric (mm) and imperial (inches) units, a unit conversion is required.

1. Calculate sidewall height in mm: multiply tread width by the aspect ratio divided by 100.
2. Convert the total sidewall contribution to inches: multiply sidewall height by 2, then divide by 25.4.
3. Add wheel diameter in inches to get total tire diameter in inches.
4. Multiply by π (3.14159) to get circumference.

Written as a formula:

Circumference (inches) = ((Tread_Width_mm × (Aspect_Ratio / 100) × 2) / 25.4 + Wheel_Diameter_inches) × 3.14159

Speed from RPM

Once circumference is known, speed in any gear follows from engine RPM, the gear ratio for that gear, and the final drive ratio.

Speed (MPH) = (Engine_RPM × Tire_Circumference_inches) / (Final_Drive_Ratio × Gear_Ratio × 1056)

The constant 1056 converts the result to miles per hour (it accounts for inches-per-mile and minutes-per-hour).

Worked Example

Using a 225/50 VR 16 tire on a 1983 944 in 5th gear at 6400 RPM:

Step 1 — Tire circumference:

• Sidewall height = 225 × (50 / 100) = 112.5 mm
• Total diameter = (112.5 × 2 / 25.4) + 16 = 8.858 + 16 = 24.07 in.
• Circumference = 24.07 × 3.14159 ≈ 75.6 inches

Step 2 — Speed:

• 1983 944 5th gear ratio: 0.8286; Final drive ratio: 3.889
• Speed = (6400 × 75.6) / (3.889 × 0.8286 × 1056) ≈ 142 MPH

Manual Calculation Reference

See TRANS-02 for the complete formula reference with formula images and an additional worked example. The gear ratio and final drive ratio values for all 924/944/968 transaxle variants are listed in TRANS-04.

Note: Many online speedometer and gear ratio calculators are now readily available and eliminate the need to perform these calculations by hand. The formulas on this page and TRANS-02 remain useful for understanding how the calculations work and for verifying calculator outputs.