Speedometer Gear Calculator

Calculate the driven gear tooth count a mechanical speedometer needs for an accurate reading, based on your tire diameter, rear axle ratio, and drive gear teeth.

Quick Facts

Formula
Driven teeth = (Drive teeth x Axle ratio x Rev/mile) / 1001
Rev/mile = 20,168 / tire diameter (inches). Standard for mechanical, cable-driven speedometers.
Calibration target
1001 cable revolutions per mile
The constant most U.S. speedometer heads are designed around when drive and driven gears are matched correctly.

Your Results

Calculated
Recommended driven gear
-
Nearest whole-tooth match
Exact calculated value
-
Unrounded driven gear teeth
Tire revolutions per mile
-
63,360 in/mi ÷ (π × diameter)
Speedometer check at 60 mph
-
Using your currently installed gear

Ready

Enter your tire diameter, axle ratio, and gear teeth, then press Calculate.

About the Speedometer Gear Calculator

A mechanical, cable-driven speedometer does not measure road speed directly — it counts how fast a small gear spins inside the transmission tailhousing. That gear (the driven gear) meshes with a drive gear fixed to the transmission's output shaft. Every time you change tire diameter or rear axle ratio, the output shaft turns at a different rate for the same road speed, so the driven gear has to change too or the speedometer (and odometer) will read the wrong number.

The standard formula

The tooth count for the driven gear is calculated in two steps, using the industry-standard 1001 calibration constant found in factory service manuals and aftermarket gear charts:

  • Tire revolutions per mile = 20,168 ÷ tire diameter (inches). This comes from a mile (63,360 inches) divided by the tire's rolling circumference (π × diameter), rounded to the constant used industry-wide.
  • Driven gear teeth = (Drive gear teeth × Axle ratio × Revolutions per mile) ÷ 1001. The 1001 divisor is the cable speed, in revolutions per mile, that most mechanical speedometer heads are designed around.

Because gears are cut with a whole number of teeth, round the result to the nearest available tooth count. This calculator also reports the reading error you would see at 60 mph if you keep a driven gear that no longer matches your tires and axle.

How to get accurate results

  • Measure the tire's actual rolled diameter (or read it off the sidewall and compute it) rather than guessing — a couple of inches changes the answer noticeably.
  • Use the final drive (rear axle) ratio, not the transmission's overall low-gear ratio.
  • Find the drive gear tooth count stamped on the gear itself or listed in your transmission's parts catalog — it is fixed by the transmission and does not change with tires or axle ratio.
  • This formula assumes a direct 1:1 output-shaft speed, which is standard for the drive gear location on GM, Ford, and Mopar automatic and manual transmissions with a mechanical speedo drive.

Practical context

This calculator gives you the arithmetic; the physical gear still has to exist in a parts catalog. If the exact calculated tooth count is not sold, pick the closest available gear and accept the small residual error, or consider an adjustable/electronic speedometer recalibrator if the mismatch is large. Vehicles with an electronic speed sensor and digital speedometer are recalibrated differently — usually in software — and this drive/driven gear formula does not apply to them.

Frequently Asked Questions

What is the standard speedometer gear formula?
Driven gear teeth = (Drive gear teeth × Axle ratio × Tire revolutions per mile) ÷ 1001, where tire revolutions per mile = 20,168 ÷ tire diameter in inches. This is the formula used industry-wide for mechanical, cable-driven speedometers on transmissions with a drive/driven gear set in the tailhousing or adapter.
Why does my speedometer read wrong after changing tires or gears?
A mechanical speedometer is calibrated for one specific combination of tire diameter, axle ratio, and driven gear tooth count. Bigger tires turn fewer times per mile, so an unchanged driven gear under-reports speed. A numerically higher axle ratio spins the driveshaft faster per mile, so an unchanged driven gear over-reports speed. Recalculating the driven gear restores an accurate reading.
What if the exact calculated tooth count isn't available?
Driven gears are manufactured in whole-tooth increments, typically from about 15 to 42 teeth depending on the transmission family, so round the calculated value to the nearest available gear. Rounding to the nearest tooth generally keeps the indicated speed within a percent or two of true speed; if no close match exists, an aftermarket electronic speedometer recalibrator or a different drive gear can close the gap.
Does this formula apply to electronic speedometers?
No. This drive/driven gear formula applies to mechanical, cable-driven speedometers. Vehicles with an electronic vehicle speed sensor (VSS) and a digital speedometer are usually recalibrated in software or with a programmer instead of a physical gear swap.

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