Find the right driven-gear tooth count for your axle ratio and tires — and check how far off your current gear reads.
Your drivetrain
Count the teeth on the small gear at the transmission output shaft — common counts: 6–9.
Overall tire height — measure it or use the tire maker’s published spec.
Share & saved setups
Worked examples
Tire revs/mile = 63,360 ÷ (π × 28) = 720.3. Driven teeth = 15 × 3.55 × 720.3 ÷ 1000 = 53.25 × 0.7203 = 38.36 → 38 teeth. With 38: cable revs = 38,356 ÷ 38 = 1,009.4 → speedo reads 0.9% fast (shows 60.6 when you’re doing 60).
Tire revs/mile = 63,360 ÷ (π × 33) = 611.2. Driven teeth = 18 × 4.10 × 611.2 ÷ 1000 = 73.8 × 0.6112 = 45.10 → 45 teeth. With 45: cable revs = 45,105 ÷ 45 = 1,002.3 → reads 0.2% fast — nearly perfect.
FAQs
Why 1000 cable revolutions per mile?
My calculated gear has a fraction — which way do I round?
What if the right tooth count isn’t sold for my transmission?
Does this work for electronic speedometers?
Which gear do I have installed now?
Why is my speedometer off after bigger tires?
Where do I find my drive gear?
The drive gear sits on the transmission’s output shaft (or the transfer case output on 4x4s), under the speedometer driven-gear housing. Remove the retaining bolt, pull the housing and driven gear, and the drive gear is visible — count its teeth directly. If it’s pressed on and hard to see, a borescope or a parts diagram for your exact transmission will identify it.
What if the exact tooth count I need isn’t sold?
Buy the nearest available whole-tooth gear and accept the small residual error — roughly the tooth difference divided by the correct count. Using 21 teeth when 21.49 is ideal leaves about 2.3% error, roughly 1.4 mph at 60 mph. If that’s too much, a different drive gear (where available) can shift the required driven count onto a manufactured size.
My vehicle has an electronic speedometer with no gears. What are my options?
Electronic speedometers are corrected with a calibration module or programmer, not gears. Inline calibrators splice into the vehicle speed sensor signal and apply a correction factor; handheld programmers (for supported trucks) rewrite the tire-size and gear-ratio values in the computer. Both still need actual tire revs per mile and axle ratio — the formulas above give you the correction percentage to program.
How much speedometer error is acceptable?
Aim for under 2–3%. Most regulations allow a speedometer to read slightly fast but never slow, and most drivers notice errors above ~3 mph at highway speed. One driven-gear tooth typically changes the reading 2–5%, so the nearest whole tooth usually lands well inside the acceptable range.
Does a wrong speedometer gear affect the odometer too?
Yes — the odometer runs off the same cable signal (or speed-sensor pulse), so it inherits the same error percentage. A speedometer reading 5% fast also racks up 5% extra miles. Correct it before selling the vehicle: odometer accuracy is a legal matter in most states.
Limitations
When you change axle gears or tire size, your speedometer reads incorrectly. A speedometer gear calculator figures out the correct driven gear tooth count for a mechanical speedometer: enter drive gear teeth, axle ratio, and tire revs per mile, and it returns the gear you need to install.
How to use the speedometer gear calculator
- Enter your drive gear tooth count. The gear on the transmission output shaft (or transfer case); common counts are 6–9 teeth. Count them yourself — don’t guess.
- Enter your axle ratio. Use the ratio actually in the differential now (e.g., 3.73) — check the axle tag or RPO label, or rotate the driveshaft and count.
- Enter tire revs per mile. If you know your tire’s published revs-per-mile figure, enter it directly. Otherwise enter the tire’s overall diameter and let the calculator derive it.
- Read the driven gear tooth count. Round the decimal result to the nearest whole tooth — that’s the gear to buy.
- Pick the closest available gear. Not every count is manufactured, so choose the nearest available and note the residual error the calculator shows.
- Verify with GPS after install. Drive at a steady 60 mph on a GPS app and confirm the speedometer agrees within a couple of mph.
Speedometer gear formulas with worked examples
Tire revs per mile
Tire revs per mile = 63,360 ÷ (π × tire diameter in inches)
Example: a 28-inch tall tire.
63,360 ÷ (3.1416 × 28) = 63,360 ÷ 87.9648 = 720.3 revs/mile.
The 63,360 is simply the number of inches in a mile. Larger tires turn fewer revolutions per mile, which is why taller tires make the speedometer read slow.
Driven gear teeth
Driven gear teeth = drive teeth × axle ratio × tire revs/mile ÷ 1000
Example: an 8-tooth drive gear, 3.73 axle ratio, and 720.3 revs/mile.
8 × 3.73 × 720.3 ÷ 1000 = 21,493.8 ÷ 1000 = 21.494 → 21 teeth (round to the nearest whole tooth).
The 1000 constant is the standard speedometer calibration figure (1000 cable revolutions per mile), built into mechanical speedometer heads.
| Drive teeth | Axle ratio | Tire diameter | Driven gear |
|---|---|---|---|
| 8 | 3.73 | 28 in | 21 teeth |
| 7 | 4.10 | 33 in | 18 teeth |
| 9 | 3.42 | 31 in | 20 teeth |
Common mistakes
- Using the wrong drive-gear tooth count. The drive gear is the small one on the output shaft, and its count varies by transmission and year. Pull it and count the teeth — guessing, or using a forum post for a different model year, ruins the whole calculation.
- Forgetting the tire size changed too. Most people regear because they fitted bigger tires, then calculate using the old tire diameter. The formula needs the tires currently on the vehicle, measured or taken from the manufacturer’s spec.
- Rounding down instead of to the nearest tooth. A result of 21.9 rounds to 22, not 21. Always round to the nearest whole tooth; rounding down systematically biases the speedometer fast.
- Not verifying with GPS after install. The calculation gets you to the right gear, but worn tires, actual versus advertised tire diameter, and gear availability all introduce small errors. A five-minute GPS check at highway speed confirms the job.
- Using the axle ratio the vehicle came with. If the previous owner regeared — or you just did — the door sticker and RPO codes may be wrong. Verify the ratio in the differential before ordering a gear.