Calculate the ideal speedometer driven gear tooth count using tire diameter, rear axle ratio, and transmission drive gear teeth. This calculator helps you correct speedometer error for many TCI and cable-driven speedometer setups.
Waiting for input Enter values and click Calculate Gear.
Formula used: Driven Gear Teeth = (Drive Gear Teeth × Axle Ratio × Tire Revs per Mile) ÷ 1000, where Tire Revs/Mile = 63360 ÷ (π × Tire Diameter in inches).
This TCI speedometer gear calculator is designed for quick, practical gear selection when you are dialing in a cable-driven speedometer. If your speedometer became inaccurate after a rear gear change, tire size swap, transmission replacement, or converter and driveline updates, the problem is usually solved by selecting the correct driven gear tooth count.
To use the calculator, enter your tire diameter, rear axle ratio, and transmission drive gear teeth. The tool returns the exact calculated driven gear count, a rounded whole-tooth recommendation, and a nearest practical result in the common 16 to 45 tooth range. If you also enter your current driven gear, the calculator estimates your speedometer error and shows your actual road speed when the gauge indicates 60 mph.
Accurate speedometer readings are important for street driving, highway cruising, and consistent tuning. A speedometer that reads high or low can lead to inconsistent shift feel perception, inaccurate mileage tracking, and poor confidence in road speed. In performance and restomod applications, accurate speed data also helps when comparing shift points, converter behavior, and cruise RPM after drivetrain changes.
Even a small mismatch in driven gear teeth can add up. For example, if your setup needs a 22-tooth driven gear but you run a 20-tooth gear, your indicated speed may be significantly off, especially at highway speeds. The TCI speedometer gear calculator helps reduce that guesswork and gives you a data-based starting point before final road validation.
The calculator uses a standard mechanical speedometer relationship: the transmission output turns the drive gear, the drive gear turns the driven gear, and the speedometer cable sees that resulting rotational speed. The target is matching cable speed to the speedometer head calibration.
Tire diameter has a direct effect because larger tires rotate fewer times per mile, while smaller tires rotate more times per mile. Rear axle ratio has a strong effect because numerically higher gears multiply output shaft turns for a given road speed. Drive gear tooth count sets the baseline ratio relationship inside the transmission speedometer gear drive.
Printed tire size can differ from loaded, real-world rolling diameter. For highest accuracy, measure from ground to tire top at normal inflation with vehicle weight on the tire, then double the center-to-ground value, or use a rollout method. This reduces error in revs-per-mile calculations.
If you are unsure whether your axle is 3.55, 3.73, 4.10, or another ratio, verify by build sheet, tag, or rotational check. Entering an assumed ratio is one of the most common sources of speedometer mismatch.
Different transmission families and housing arrangements may use different drive gear counts. If you can verify actual installed teeth, calculator output will be much more reliable. When in doubt, reference the transmission documentation for your exact model and speedometer gear housing.
Entering your current driven gear provides immediate insight: you can see whether your speedometer reads high or low and by about how much. This is useful when choosing between two nearby gear tooth options that are both physically available.
The exact output can include decimals, but real gears are whole-tooth parts. Start with the rounded recommendation, then choose the nearest available part in your compatible gear family. If exact calibration is critical, road test with GPS speed and make a one-tooth adjustment as needed.
If your speedometer reads too high, you usually need more driven gear teeth. If it reads too low, you generally need fewer driven gear teeth. The error estimate in this calculator helps you identify correction direction quickly.
| Change Made | Likely Speedometer Effect | Typical Correction Direction |
|---|---|---|
| Installed taller tire diameter | Speedometer often reads lower than actual | Use fewer driven gear teeth |
| Installed shorter tire diameter | Speedometer often reads higher than actual | Use more driven gear teeth |
| Swapped to numerically higher axle ratio (e.g., 3.23 to 3.73) | Speedometer often reads higher than actual | Use more driven gear teeth |
| Swapped to numerically lower axle ratio (e.g., 4.10 to 3.55) | Speedometer often reads lower than actual | Use fewer driven gear teeth |
This calculator is intended for mechanical gear-driven speedometer systems. If your build uses an electronic sender and programmable speedometer, correction may be done in software or through pulse-per-mile settings rather than a physical driven gear swap. Hybrid setups exist, so always confirm whether your speed signal is mechanical cable, sensor-based, or converter-box controlled.
Use the calculator to select a first-pass driven gear, then road test at 30, 45, 60, and 75 mph against GPS. If error is consistent, one-tooth changes usually provide predictable improvements. Keep notes of tire pressure, fuel load, and test route grade for repeatable comparisons. Precision calibration is easiest when drivetrain, tire pressure, and ride height are finalized.
For performance street cars and long-distance cruisers, a well-calibrated speedometer improves confidence and helps prevent accidental speeding. For restoration-grade builds, it improves period-correct usability while preserving mechanical authenticity.
Measure actual installed diameter with vehicle weight on the tires, or calculate revs-per-mile using a rollout method. Real-world diameter can differ from nominal labeled size.
Yes, gears are whole-tooth parts. Start with the nearest whole number, then road test and adjust by one tooth if needed.
Possible causes include incorrect drive gear count, tire diameter mismatch, mechanical cable issues, speedometer head calibration drift, or incompatible gear housing parts.
Yes. The math applies broadly to many mechanical cable-driven systems that use transmission drive and driven speedometer gears.
It depends on the target tooth count and overall setup, but one tooth can create a noticeable change. Use GPS to verify after each adjustment.
A drivetrain change should not leave your speedometer guessing. This TCI speedometer gear calculator gives you a clear, repeatable method to choose the right driven gear tooth count and estimate current error before you buy parts. Enter accurate inputs, select the nearest available gear, validate with GPS, and fine-tune by one tooth if needed. That process delivers reliable, road-true speed readings for street, strip, and long-haul driving alike.