Find the correct driven gear tooth count for a cable-driven speedometer after a tire or axle-gear change — from drive teeth, axle ratio and tire revs/mile. Formula: Driven = (Drive × Axle × Revs/Mile) ÷ 1001 (TCI Automotive).
Mode 1 — Calculate driven gear teeth
Mode 2 — Reverse: error of the installed driven gear
Already have a driven gear installed? Enter your setup to see the speedometer percentage error and an indicated-vs-actual speed table.
Mode 3 — TCI speedometer gear charts
Manufactured drive & driven gear tooth counts per transmission, transcribed from TCI Automotive's published speedometer gear charts (colors are the factory color codes). The drive gear is on the transmission output shaft; the driven gear lives in the removable housing and drives the cable.
Source: TCI Auto speedometer gear calculator page. Notes: TH350/TH400/700R4 — gears with 34+ teeth need the larger 2.078" housing. Torqueflite housing has three orientation positions: 25–31, 32–38, 39–45 teeth. Ford 23-tooth driven gear is a limited-life retrofit for 3.73/4.10 axle gears. Ford drive gears are machined into the output shaft.
Related tools & boundaries. This calculator computes physical speedometer gear teeth (drive/driven gears).
Tire size comparator only reports the percentage speedometer error from a tire-size change (no gear teeth).
Gear ratio calculator covers transmission/axle gear ratios, not speedometer gears.
How it works
A cable-driven speedometer is calibrated for about 1001 cable revolutions per mile. The cable spins at drive teeth × axle ratio × tire revs/mile ÷ driven teeth revolutions per mile. Setting that equal to 1001 and solving gives the TCI formula used above. Tire revs/mile come straight from the tire circumference: 63360 ÷ (π × diameter) = 20168 ÷ diameter.
Assumptions: revs/mile from the nominal diameter is theoretical — a loaded, rolling tire (deflection, pressure) differs slightly. One gear tooth changes the reading by roughly 1 ÷ teeth (a few percent), so residual error after rounding to a manufactured gear is normal. Modern vehicles with electronic vehicle-speed sensors have no speedometer gears at all.
Frequently asked questions
What is a speedometer gear, and what is the difference between drive and driven gears?
On older cable-driven speedometers, two small gears translate driveshaft rotation into cable rotation. The drive gear sits on the transmission output shaft; the driven gear sits in a removable housing (usually in the tailhousing) and connects to the speedometer cable. When you change tire size or axle ratio, the driveshaft spins at a different rate per mile, so you must change the driven gear tooth count to keep the speedometer accurate.
What is the formula for speedometer gear teeth?
The industry-standard formula (published by TCI Automotive) is: Driven Teeth = (Drive Teeth × Axle Ratio × Tire Revs per Mile) ÷ 1001, where Tire Revs per Mile = 20168 ÷ Tire Diameter in inches. The constant 1001 comes from the speedometer cable calibration (about 1000 cable revolutions per mile). Some field references use 1000 instead of 1001 — the difference is 0.1%, negligible next to one gear tooth.
What speedometer gear do I need after swapping from 3.08 to 3.73 gears with 33-inch tires?
Worked example with an 8-tooth drive gear (TH350): tire revs per mile = 20168 ÷ 33 = 611.15. Driven teeth = (8 × 3.73 × 611.15) ÷ 1001 = 18.22. The closest manufactured TH350 driven gear is 18 teeth, leaving a residual error of (18.22 ÷ 18 − 1) × 100 = +1.21% — the speedometer reads about 1.2% high. Enter your own numbers in Mode 1 above.
How much does one tooth change the speedometer reading?
Roughly 1 ÷ (driven teeth) × 100 percent. On an 18-tooth driven gear, going up one tooth (18 → 19) changes the reading by (18 ÷ 19 − 1) × 100 ≈ −5.3%. Fewer teeth make the cable spin faster, so the speedometer reads higher; more teeth slow it down.
Why is my calculated tooth count not a whole number?
Gears are only manufactured in whole tooth counts, so the exact value (e.g. 18.22) must be rounded to a gear that actually exists. The calculator picks the closest manufactured driven gear for your transmission (from TCI's published charts) and shows the residual error — for example +1.21% when using an 18-tooth gear for an 18.22 calculation.
How do I check the error of the driven gear I already installed?
Use Mode 2 (reverse): enter your drive teeth, axle ratio, tire size and the installed driven teeth. Example: 8-tooth drive, 3.73 axle, 33-inch tires, 18-tooth driven gear installed gives (18.22 ÷ 18 − 1) × 100 = +1.21% error — when the speedometer shows 60 mph you are actually doing 59.3 mph. The mode also prints a full indicated-vs-actual speed table.
How do I find my drive gear tooth count?
The drive gear is on the transmission output shaft inside the tailhousing. On many GM transmissions you can read the tooth count (and color code) once the housing is off; TCI's charts list the factory drive-gear options per transmission (e.g. TH350: 7, 8, 9, 10, 17, 18, 19, 20 teeth). On Ford C3/C4/C5/C6 and AOD transmissions the drive gear is machined into the output shaft, so you may need to remove the tailhousing to count the teeth.
Do I need a different driven-gear housing for some tooth counts?
Sometimes. TCI notes that some transmissions use different driven-gear housings depending on tooth count — for example TH350/TH400/700R4 gears with 34 or more teeth need a larger 2.078-inch housing, and Torqueflite housings have three orientation positions covering 25–31, 32–38 and 39–45 teeth. Check your transmission's notes in the gear chart (Mode 3).
How do I get tire revolutions per mile from my tire size?
Divide 20168 by the tire's overall diameter in inches. A 33-inch tire gives 20168 ÷ 33 = 611.15 revs/mile. For metric sizes, first compute the diameter: rim inches + 2 × width_mm × (aspect ÷ 100) ÷ 25.4 — e.g. 205/55R16 = 24.88 in diameter, so 20168 ÷ 24.88 = 810.68 revs/mile. For flotation sizes like 33x12.50R17, the leading number is the nominal diameter. This is the theoretical value; a loaded, rolling tire can differ slightly.
My car has an electronic speedometer. Do I need gears?
No. Vehicles with electronic speed sensors (VSS) have no physical speedometer gears — calibration is done with a programmer, tuner, or dealer tool that adjusts pulses-per-mile. This calculator is for classic cable-driven speedometers found in older GM, Ford and Chrysler vehicles.
How is this different from a speedometer error calculator or a gear ratio calculator?
A speedometer error calculator (like our tire size comparator) only tells you the percentage error caused by changing tire size. A gear ratio calculator deals with transmission and axle ratios. This tool goes one step further: it computes the physical driven-gear tooth count you must install to correct the speedometer after a tire or axle change.
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