SPEEDOMETER DRIVE GEARS
Off-site Gear Calculator

FORD TOPLOADER, T-5 AND TREMEC

This table assumes that the transmission uses the common 7 tooth drive gear, and is applicable to most Ford Toploader, T5 and Tremecs.  For the Richmond Gear 5 speed, see below.  Determine your tire size here.

 

DIFFERENTIAL RATIO

TIRE DIAM.

2.88

3.07

3.31

3.54

3.77

25

16.3

17.3

18.7

20.0

21.3

25.5

15.9

17.0

18.3

19.6

20.9

26

15.6

16.7

18.0

19.2

20.5

26.5

15.3

16.4

17.6

18.9

20.1

27

15.1

16.1

17.3

18.5

19.7
Number of teeth on driven gear

Obviously some compromise must be made: The gears only come in even numbers! There will be some tire slip but even so, using the next lower even tooth value will usually result in a close but slightly "optimistic" speedometer reading.

Ford part numbers and color codes
Teeth

LH spiral
(Toploader)

Color

 Teeth

RH spiral
(T-5, Tremec)

Color
17 C2DZ-17271G Purple tip 17 C3DZ--17271C White body
18 C2DZ-17271K Green tip 18 C0DD-17271B Yellow body
19 C4DZ-17271A Pink tip 19 C0DZ-17271B Pink body
20 C2DZ-17271H Blue tip 20 C8SZ-17271B Orange body

Drive Gears
Toploader (LH spiral) T-5, Tremec (RH spiral)
C4DZ-17285A Black C8AZ-17285A Yellow

RICHMOND GEAR 5 SPEED

This transmission commonly uses an 8 tooth drive gear.

 

DIFFERENTIAL RATIO

TIRE DIAM.

2.88

3.07

3.31

3.54

3.77

25

18.6

19.8

21.4

22.8

24.3

25.5

18.2

19.4

20.9

22.4

23.9

26

17.9

19.1

20.5

22.0

23.4

26.5

17.5

18.7

20.2

21.6

23.0
Number of teeth on driven gear

Obviously some compromise must be made: The gears only come in even numbers!  There will be some tire slip but even so, using the next lower even tooth value will usually result in a close but slightly "optimistic" speedometer reading.

GM driven-gear part numbers
and color codes

Teeth

Part number

Color

18

3987918

Brown

19

3987919

White

20

3987920

Blue

21

3987921

Red

22

3987922

Gray