Monster Speedometer Gear Calculator

Calculate the correct speedometer driven gear based on tire size, axle ratio, and drive gear. Built‑in P‑metric converter.

Driveline

Drive Gear Teeth (output shaft) Axle Ratio Transmission (for range check) Current Driven Gear (optional)

Tires

Tire Diameter (in)

Tire Width (mm) Aspect Ratio (%) Wheel Diameter (in)

±2% → Excellent

2–5% → Acceptable

5%+ → Consider changes

Results

Tire Revs / Mile—

Ideal Driven Gear (exact)—

Suggested Driven Gear (nearest)—

Est. Speedometer Error (with suggested)—

Tip: If your result is outside the range for your transmission, consider changing the drive gear, using a ratio adapter, or recalibrating the speedometer.

Speedo Gear FAQ

My transmission is electronic (VSS). Will this work?

Electronic units like 4L60E (’93+), 4L80E, 6L80/90, and modern Ford/Chrysler use a sensor and require a tune, programmer, or signal adapter — not gears.

Where do I find my drive gear tooth count?

On most GM gear‑driven units, the drive gear is on the output shaft (inside the tail housing). Count teeth at service or reference the build sheet. 700R4 typically uses 13/15/17.

What driven gear housings exist for GM?

Common ranges are 34–39 and 40–45. Stay within the matching housing range for proper mesh. Outside that, change the drive gear or use a ratio adapter.

Should I round up or down on the driven gear?

Round to the nearest whole tooth. The calculator shows the estimated error for the rounded choice and for your current gear if you enter it.

My speedo is still off. What else affects it?

Actual tire diameter varies by brand/load, and older speedometers can drift. Verify tire diameter by rolling circumference, and check the speedo head or cable condition.

Speedometer Gear Charts

GM — 2004R

Drive Gears	Color
10 teeth	Green
11 teeth	Orange*
12 teeth	Red
13 teeth	White

Driven Gears	Color
26 teeth	Brown
27 teeth	Black
28 teeth	Yellow
29 teeth	Green
30 teeth	Blue
31 teeth	White

All 2004R use a 1" steel driven gear housing. Drive gear is part of the governor (inside pan).

GM — TH350

Drive Gears	Color
7 teeth	Pink
8 teeth	Black
9 teeth	Green
10 teeth	Red or Purple

Driven Gears	Color
18 teeth	Brown
19 teeth	Natural
20 teeth	Blue
21 teeth	Red
22 teeth	Gray
34 teeth	Light Green
35 teeth	Orange/Pink
36 teeth	White
37 teeth	Red
38 teeth	Blue
39 teeth	Brown
40 teeth	Black
41 teeth	Yellow
42 teeth	Green
43 teeth	Purple
44 teeth	Dark Gray
45 teeth	Light Blue

Chevy = 1" housing in tailhousing (2WD). BOP = 2.078" housing; separate housings for 34–39 vs. 40–45 tooth gears.

GM — TH400

Drive Gears	Color
7 teeth	Red
8 teeth	Black
15 teeth	Brown or Gray*
16 teeth	Orange
17 teeth	Red
18 teeth	Yellow or Blue
19 teeth	Yellow or White
20 teeth	Green
21 teeth	Black

Driven Gears	Notes / Color
16	Truck
17	Truck
18	Truck (code H)
19	Truck (code J)
20	Truck (code K)
21	Truck (code L)
22	Truck (code M)
34	Light Green
35	Pink
36	White
37	Red
38	Blue
39	Brown
40	Black
41	Yellow
42	Green
43	Purple
44	Dark Gray
45	Light Blue

Two aluminum housings for 34+ teeth (34–39 and 40–45). Driven housings sit just forward of the tailhousing.

GM — 700R4

Drive Gears	Color
15 teeth	Gray
17 teeth	Red*
18 teeth	Blue
19 teeth	Yellow
20 teeth	Brown

Driven Gears	Color
34	Light Green
35	Orange/Pink
36	White
37	Red
38	Blue
39	Brown
40	Black
41	Yellow
42	Green
43	Purple
44	Dark Gray
45	Light Blue

Two 2.078" housings (aluminum/plastic) for 34–45 teeth. Driven housings in the tailhousing (2WD).

Chrysler — Torqueflite

Drive Gear	Location
13 teeth	Machined into output shaft

Driven Gears	Color
25	Light Green
26	Red
27	Beige
28	Light Blue
29	Black
30	Yellow
31	Green
33	Yellow
34	Green
35	Orange
36	Red
39	Green
40	Orange
41	Red
45	Yellow

Uses 1.75" driven gear housing in tailhousing (2WD). Set housing orientation by tooth range: 25–31 / 32–38 / 39–45.

Ford

Drive Gears	Notes
6 teeth	C6 only
7 teeth	Machined into output shaft
8 teeth	Machined into output shaft
9 teeth	Machined into output shaft

Driven Gears	Color
16	Tan
19	Tan
20	Orange
21	Red
23	White¹

Drive gears are machined into the output shaft on C3/C4/C5/C6 & AOD. Tailhousing removal may be needed to count teeth.
1. 23-tooth white = retrofit for 3.73 & 4.10; service life typically < 25,000 miles.