Formula
A cable speedometer is turned by a pair of gears. The drive gear is on the transmission output shaft. The driven gear is in the tailhousing, or, on a TH400, in the case just ahead of it. Tire revolutions per mile times the axle ratio is how fast that output shaft turns. Driven teeth are that speed times the drive teeth, divided by the revolutions the speedometer head expects in a mile.
revs/mile = 63,360 / (π × diameter)
output = revs/mile × axle
driven = output × drive teeth / speedometer revs
indicated = true × (output × drive / driven) / speedometer revs
The diameter formula is 63,360 inches in a mile. It is not the rounded 20,168 / diameter shortcut. A growth factor multiplies the diameter before that division, for a loaded radius. If you type the tire maker’s revolutions per mile, the page uses that number and ignores the growth factor, because the spec sheet already includes the load. 275/60R15 is 27.99 inches by the same sidewall formula as the tire size page.
GM cable speedometers are usually 1,001 revolutions per mile. Ford and Chrysler cable speedometers are usually 1,000, which is also the divisor on Ford Racing’s chart. The field is editable because the head on the car is the one that counts. The nearest whole tooth, and one tooth either side, are shown with the percent error and the indicated speed at a true 60 mph. More driven teeth slow the cable, so the speedometer reads low.
A tire-only check does not need gears. If the speedometer was calibrated to the old diameter, the indicated speed is the true speed times the old diameter divided by the new one. That is the opposite direction from “how fast am I really going.” Odometer error equals speedometer error, because they share the cable.
Electronic transmissions, including the 4L60E, do not take these gears. The vehicle speed sensor is calibrated in the computer. This page will not invent a tooth count for that case. It will still show the percent a tire change creates.
Worked examples
27 inch tire, 3.73 axle. 746.97 revolutions per mile. Output shaft 2,786.19. An 8-tooth drive at 1,001 lands on 22.27 teeth, nearest 22. A 15-tooth drive lands on 41.75, nearest 42. A 17-tooth drive lands on 47.32, which is outside the 700R4 driven list. A 15-tooth drive on that same tire is the chart combination.
31 inch tire, 4.10 axle. About 650.58 revolutions per mile. A 15-tooth drive is 39.97 teeth, nearest 40. An 8-tooth drive is 21.32, nearest 21. Left on a 15-tooth drive and a 41-tooth driven gear, the cable turns about 975.9 times a mile and the speedometer reads about 58.5 mph at a true 60.
Calibrated at 27 inches, now on 26. Indicated speed is 62.3 mph at a true 60.
275/60R15. 27.99 inches.
Reference
Drive and driven teeth below are the published lists this page will check. A blank range was not filled in. Colors on a maker’s chart identify a gear; count the teeth.
| Transmission | Drive teeth | Driven teeth | Speedo |
|---|---|---|---|
| GM 700R4 / 4L60 | 15, 17–20 | 18–24, 34–45 | 1001 |
| GM TH350 | 7–10, 17–20 | 18–22, 34–45 | 1001 |
| GM TH400 | 7–8, 15–21 | 16–22, 34–45 | 1001 |
| GM Muncie / Saginaw | Not locked | Not locked | 1001 |
| Ford C4 / C6 / AOD | 6–9 | 16, 19–21, 23 | 1000 |
| Chrysler 727 / 904 | 13 | 25–31, 33–36, 39–41, 45 | 1000 |
| Electronic (4L60E and later) | Not a gear | Calibrate in the ECU | — |
Questions
What speedometer gear do I need?
Driven teeth equal tire revolutions per mile, times the axle ratio, times the drive-gear teeth, divided by the speedometer’s revolutions per mile. A 27 inch tire and a 3.73 axle turn the output shaft about 2,786 times a mile. A 15-tooth drive and a 1,001-revolution GM speedometer need 41.75 driven teeth, so the nearest gear is 42. Count the gear that is in the transmission before you order against a chart.
How do I tell the drive gear from the driven gear?
The drive gear is on the output shaft. The driven gear is the one the cable or the sensor is turned by, usually in the tailhousing. On a TH400 the larger driven gears sit in the case just ahead of the tailhousing. Tooth count is often stamped or printed in a color on a chart. Count the teeth anyway.
What are the 700R4 speedometer gear options?
TCI Auto’s chart lists drive teeth 15, 17, 18, 19, and 20 for the mechanical 700R4 and 4L60. Driven teeth are 18–24 and 34–45. There is no driven gear from 25 through 33. Two 2.078 inch housings cover 34–39 and 40–45. The 4L60E does not use these gears.
Why does the speedometer read high after smaller tires?
The speedometer still thinks each revolution covers the old, taller distance. A speedometer calibrated at 27 inches and now on 26 inch tires reads about 62.3 mph at a true 60. The indicated speed is true speed times the old diameter divided by the new diameter.
Can a GPS check the speedometer?
Yes. Drive at a steady speed and compare the speedometer with a GPS. Do that after the gears are in. An inaccurate speedometer can lead to speeding tickets and odometer errors. The odometer is off by the same percent as the speedometer.
How is an electronic speedometer calibrated?
4L60E and later transmissions, and other gearboxes with a vehicle speed sensor, are calibrated in the ECU or a programmer. This page will not recommend a tooth count for those. It will still show the percent a tire change throws the reading off.
Does speedometer error change the odometer?
Yes. On a cable speedometer the odometer is driven by the same cable. If the speedometer reads 2% low, the odometer records about 2% fewer miles. A GPS check covers both.
Sources
- TCI Auto, speedometer gear calculator chart, tciauto.com/speedometer-gear-calculator. Drive and driven tooth lists for the 700R4 / 4L60, TH350, TH400, Ford C4/C6/AOD, and Chrysler 727/904 are the discrete sets that chart prints. Gaps, including 25–33 on the 700R4 and the missing Torqueflite teeth, are left empty. Housing splits are described the way that chart describes them. Citing the chart is not a recommendation to buy a gear.
- Ford Racing, Speedometer Gear Usage Chart. That chart divides by 1,000 and notes that most automatic drive gears are machined into the output shaft.
- Chrysler driven teeth 25–31, 33–36, 39–41, and 45 are the TCI list as reprinted in full by Monster Transmission’s Torqueflite gear table. Tooth 29 is on that reprint. Teeth 32, 37, 38, 42, 43, and 44 are not.
- Muncie and Saginaw tooth counts are editable and are not range-checked. A GM or gear-maker chart for those boxes was not locked.
- Revolutions per mile use 63,360 / (π × diameter), the same identity as the tire size calculator. A tire maker’s loaded revolutions-per-mile sheet is the better input when you have it. Engine RPM is the gear ratio calculator. Wheel position is the wheel offset calculator.