T5 Speedometer Gear Calculator
Changing rear gears or tire size on your Mustang often throws off the speedometer. Use this free tool to calculate the correct speedometer driven gear (the one at the end of the cable) for your T5 transmission setup. It also helps you identify your internal drive gear to ensure accuracy.
Note: Always verify calculations independently before purchasing parts. Results are estimates and may vary based on actual tire dimensions, manufacturing tolerances, and wear.
Vehicle Details
Don't know your ratio? Look it up by your axle tag
Tire Size
Calculated Diameter: "
Changing tires? Compare it with another size
Results
T5 Speedometer Gear Formula
The calculator uses the Ford speedometer gear formula:
driven gear teeth = drive gear teeth × axle ratio × tire revolutions per mile ÷ 1,000
Tire revolutions per mile is 63,360 (inches in a mile) divided by the tire's circumference (diameter × π). The 1,000 is the speedometer's constant: the cable turns 1,000 times per mile. The answer is rarely a whole number, so the calculator picks the driven gear with the closest tooth count and shows how far off the speedometer will read.
Example (the calculator's defaults): a 7-tooth drive gear, a 3.55 axle and 225/60R15 tires (25.63" tall). 63,360 ÷ (25.63 × π) = 786.9 revolutions per mile, and 7 × 3.55 × 786.9 ÷ 1,000 = 19.55 teeth. The closest driven gear is the 20-tooth (Black), which reads 2.3% slow: at 60 mph the speedometer shows 58.6 mph.
Ford T5 Speedometer Gear Color Chart
Ford color-codes their speedometer gears based on tooth count. Whether you have a plot-in (cable) or electronic sensor setup, the gear colors remain consistent for the Type 17271 gears used in T5 and C4 transmissions.
| Tooth Count | Color | Buy |
|---|---|---|
| 16 | Wine | Amazon: 16-tooth wine gear (paid link) |
| 17 | White | 4-gear set on Amazon: 16, 17, 18 and 20 teeth (paid link) |
| 18 | Yellow | Amazon: 18-tooth yellow gear (paid link) |
| 19 | Pink | |
| 20 | Black | Amazon: 20-tooth black gear (paid link) |
| 21 | Red | |
| 23 | White | Amazon: 23-tooth white gear (paid link) |
Amazon links are paid links.
How to Identify Your T5 Drive Gear
The Drive Gear is located inside the transmission on the output shaft. You cannot change this without removing the tail shaft housing. Knowing which one you have is critical for the calculator accurately.
- 7-Tooth (Yellow): The most common gear for 1983-1989 V8 World Class T5s.
- 8-Tooth (Green): Standard on 1990-1995 V8 World Class T5s. Also common on aftermarket "Z-Spec" T5s.
- 6-Tooth (Black): Typically found in 4-cylinder T5s or early non-WC units.
Frequently Asked Questions
If your speedometer reads faster than your actual speed, your driven gear likely has too few teeth. For example, if the calculator calls for a 20-tooth gear and you have a 19-tooth installed, the cable spins faster than proper calibration, causing a high reading. You need a gear with higher tooth count to slow it down.
For standard Ford housings, 21 teeth (Red) is the practical limit for longevity. There is a 23-tooth (White) gear available (popular in Jeep circles but fits Ford), but the teeth are very thin and prone to stripping quickly ("chewed up" gears). If you need a 23-tooth gear, we highly recommend changing your internal drive gear to a 7-tooth or 6-tooth instead to bring the requirement back down to the durable 17-21 range.
Yes, it should. The Ford speedometer gear formula and the "Type 17271" driven gears are the same for the C4, T5, Tremec 3550/TKO, and AOD transmissions. However, the drive gear tooth counts inside those transmissions may vary from the T5 defaults listed above.