Speedometer Gear Calculator
Input your tire diameter, axle ratio, and transmission drive gear to find the correct driven gear and ensure accurate speedometer readings.
Use a different factor if specified by your transmission manufacturer.
How to Use This Calculator
Measure tire diameter
Use the effective rolling diameter of the tire, ideally measured under vehicle weight.
Enter axle ratio and drive gear teeth
Use the rear differential ratio and count the teeth on the transmission’s speedometer drive gear.
Use the correct speedometer factor
Most GM/Chrysler mechanical speedometers use 1,000. Check service manuals for other transmissions.
Compare to your current driven gear
Enter the existing driven gear teeth to see the resulting speedometer error and pick the closest available gear.
Formula
Driven Teeth = (Drive Teeth × Axle Ratio × Tire Revs per Mile) ÷ Speedometer Factor
Tire Revs per Mile: 20168 ÷ Tire Diameter (inches)
Example: 28" tire, 3.73 axle, 15-tooth drive gear, factor 1,000:
Tire revs/mile = 20168 ÷ 28 = 720.3
Driven teeth = (15 × 3.73 × 720.3) ÷ 1000 = 40.3 → 40 teeth recommended
About the Speedometer Gear Calculator
Swapping axle gears or tire sizes alters the relationship between driveshaft speed and vehicle speed. This calculator helps you choose the correct speedometer driven gear to keep your gauge accurate.
When to Use This Calculator
- Gear ratio change: After installing new differential gears, recalculate driven gear teeth.
- Tire upgrades: Larger or smaller tires require speedometer recalibration.
- Transmission swaps: Ensure donor transmissions match your vehicle’s speed sensor requirements.
- Restorations: Verify factory specifications during vehicle rebuilds.
Why Use Our Calculator?
- ✅ Precision: Uses standard industry constants and allows custom factors.
- ✅ Error estimation: Compares current and recommended gear counts with speed error feedback.
- ✅ Quick adjustments: Fast recalculations when testing multiple gear combinations.
- ✅ Mobile friendly: Ideal for shop use or on the track.
Common Applications
Muscle car builds: Calibrate classic GM/Chrysler mechanical speedometers.
Off-road rigs: Correct speed readings after installing oversized tires.
Transmission rebuilders: Supply accurate driven gears with gear packages.
Tips for Best Results
- Use the actual measured tire diameter; published sizes can differ once mounted.
- Speedometer gears only come in whole teeth—pick the closest available and note the small error.
- Some transmissions support only a limited range of driven gears—double-check compatibility.
- For electronic speedometers, use this calculation to select the correct calibration module or pulse rate.
Frequently Asked Questions
What if the recommended gear isn’t available?
Choose the closest tooth count and note the error percentage. Some builders use ratio adapters to fine-tune readings when gears alone can’t achieve perfection.
What is the speedometer factor?
Mechanical GM and Chrysler transmissions typically use 1,000. Ford uses 1,008. Check documentation for your transmission or speed sensor if you’re unsure.
Does this apply to electronic speedometers?
The same principles apply. Instead of swapping gears, you might adjust a calibration module or PCM setting using the calculated ratio.
How accurate is the tire revs constant?
20168/diameter is a widely used approximation. For exact accuracy, measure tire revolutions per mile using GPS or manufacturer data.