Speedometer Gear Calculator

Enter your Drive Gear Teeth, Axle Ratio, Transmission Type, and tire specs (Width, Aspect Ratio, Wheel Diameter) into the Speedometer Gear Calculator to find your Suggested Driven Gear, Tire Diameter, Revolutions per Mile, and Speedometer Error.

Number of teeth on the speedometer drive gear

Rear end gear ratio

Select your transmission for gear range verification

Leave blank if unknown

in

Overall tire diameter in inches

mm

Tire width in millimeters (for P-metric calculation)

%

Sidewall height as percentage of width

in

Rim diameter in inches

Results

Suggested Driven Gear

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Calculated Tire Diameter

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Ideal Driven Gear (Exact)

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Tire Revolutions per Mile

--

Est. Speedometer Error

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Speedometer Accuracy

Frequently Asked Questions

Will this calculator work for electronic transmissions with VSS?

No, electronic transmissions like 4L60E (1993+), 4L80E, 6L80/90, and modern Ford/Chrysler units use electronic speed sensors instead of mechanical gears. These require a programmer, tune, or signal adapter to correct speedometer readings.

Where can I find my drive gear tooth count?

The drive gear is located on the transmission output shaft where the speedometer cable or housing connects. You may need to remove the speedometer housing or cable to count the teeth. Common counts are 15, 17, 18, or 20 teeth.

Should I round up or down when choosing the driven gear?

Generally, it's better to round to the nearest whole number. If you're between two gears, consider your driving habits - round down for mostly highway driving (slight overread is safer) or round up for city driving.

My speedometer is still inaccurate after changing gears. What else could affect it?

Other factors include worn speedometer cables, incorrect odometer gears, transmission modifications, or speedometer head calibration issues. Also verify your actual tire diameter matches the calculated diameter.

What if my calculated gear is outside my transmission's range?

If the result exceeds your transmission's gear range, consider changing the drive gear, using a speedometer ratio adapter, or having the speedometer recalibrated. Some transmissions have limited driven gear ranges.

How accurate should my speedometer be?

Within 2% is excellent, 2-5% is acceptable for most applications. Errors above 5% should be corrected as they can affect fuel economy calculations and may not meet legal requirements in some areas.

Can I use P-metric tire specifications instead of measuring diameter?

Yes, enter your tire width (mm), aspect ratio (%), and wheel diameter (inches) to automatically calculate the overall tire diameter. This is often more accurate than measuring a mounted tire.

Do I need to recalibrate after changing tire sizes?

Yes, any significant change in tire diameter will affect speedometer accuracy. Even switching from one tire brand to another with the same size designation can require recalibration due to manufacturing differences.

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