Speedometer Gears for a T5 Conversion

When swapping a T5 transmission out of a fox body Mustang into an early Mustang (65-66 in my situation), the speedometer drive gear will likely need to be replaced in order to maintain an accurate speed displayed on the dash speedometer. After much searching, I found a web page that actually listed the calculation:

Speedometer Gear Ratio = ((rear axle ratio)*63360) / ((Distance car travels with one tire revolution)*1000)

The number "63360" refers to the number of inches in a mile. The distance the car travels with one tire revolution should probably be in inches. Using the ratio you derive from the calculation, use the below chart to determine which gear you need. The first number is the number of teeth on the driven gear while the second number is the number of teeth on the drive gear inside the transmission.

Gear Combination (Driven #/Drive #)	Ratio
16/8	2.0000
17/8	2.125
18/8	2.25
16/7	2.2857
19/8	2.375
17/7	2.4286
20/8	2.5
18/7	2.5714
21/8	2.625
16/6	2.6667
19/7	2.7143
17/6	2.8333
20/7	2.8571
18/6 or 21/7	3.0000
19/6	3.1667
20/6	3.3333
21/6	3.5

The drive gear inside the transmission will be one of two depending on the production year of the T5.

1983-1989= 7 tooth (Yellow- E3ZZ-17285-A)
1990-1995= 8 tooth (Green-   F0ZZ-17285-A)

Manual T5			Automatic
Teeth	Part Number	Color	Teeth	Part Number	Color
16	C0DZ-17271-A	Wine	16	D0AZ-17271-A	Blue
17	C3DZ-17271-C	White	17	C7SZ-17271-A	Green
18	C0DD-17271-B	Yellow	18	C7SZ-17271-B	Gray
19	C0DZ-17271-B	Pink	19	C7VY-17271-A	Tan
20	C1DZ-17271-A	Black	20	C8SZ-17271-B	Orange
21	C4OZ-17271-A	Red	21	D0OZ-17271-B	Purple
23	DFM-17271-A	White

Another web page I found (FFcars.com) listed a calculator in one of the posts for easier calculation. The link is HERE. The bottom line is that in order to figure out how many teeth should be on the driven gear, you need to know the number of teeth used on the drive gear (7 or 8), the rear gear ration (i.e. 3:55), and the height of the tire or the distance it travels in one revolution. The calculation on this web page is as follows:

driven gear teeth = (drive gear teeth * axle ratio * (63360/(3.14*D))/1000

Gear Calculator on Moldy Rabbit

Of course the Moldy Rabbit website should only be used for cars that are retaining the C4 automatic transmission and have simply swapped their rear gears. I didn't realize that and purchased the purple 21 tooth speed gear (DO0Z-17271-A) instead of the red speedo gear for the T5 (C4OZ-17271-A). All's well that ends well. The gear cost me less than $10 with shipping and I simply purchased the correct one new from Ebay for less than $10 as well.

I'll be running 17x8" Torq Thrust 2.1's with 245/45/17 rims on the rear. You can see the entry where I test fitted the rims HERE.