Speedometer Gears

6/19/08

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One of the more common questions on the Mustang tech boards is "What speedometer gear do I need to go with my new 3.73:1 gears?". Here's a simple calculator I drummed up to solve the problem. Enter the data from your drive line setup and hit 'Go Figure'.

Transmission Type

Rear Axle Ratio

Actual Tire Diameter [To the nearest 1/4 inch is good enough.]

Note: If you're not sure of your actual tire diameter, check out the home page of your tire manufacturer. Most manufacturers have an on-line tire selector that will give you the actual measured tire diameter. For example, my Dunlop D40M2 235/60VR15's have a measured diameter of 26.2 inches while my Mickey Thompson 26 x 8.5-15 have a running (grown) diameter of 26.0 inches.

Speedometer Drive Gear Teeth

Note: The production T5 transmissions for 1990-95 5.0L models use an 8 tooth drive gear as did the T-45 transmissions used in the 1996-98 Mustang GT. Most 1983-89 T5 transmissions used a 7 tooth drive gear as did the T-45 transmission used in the 1996-98 Mustang Cobras. Most AOD transmissions used either the 7 or 8 tooth drive gear.

Here's the part numbers for the various driven gears (courtesy of the 2003 Ford Racing Performance Parts catalog):

Manual			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

* Available from Mustangs Unlimited @ 1-800-243-7278

I've yet to see a 22 tooth driven gear available for the T5, but I've been told that Mopar dealers may have them. Refer to our Gear Swap page for details on installing the driven gear in a T5. The 23 tooth driven gears are notorious at being short lived. Mine lasted only months before stripping all the teeth of itself. I think the root diameter is so small (in order to fit so many teeth on it), that it creates a bad mesh with the drive gear. The 16 and 18-tooth driven gears for the AOD transmissions have been discontinued as of 2002. However, your local Ford parts counter may still be able to get them from stock while quantities last.

Unfortunately, the drive gear on the automatic transmissions (AOD) is integrally machined into the output shaft. This means that you'd have to replace the entire output shaft to change your drive gear to obtain the correct drive/driven gear ratios. This is pretty expensive and impractical. In this situation, you're better off obtaining a speedometer correction box to correct the speedometer readings. Check your local Yellow Pages for a speedometer shop or check with some local transmission shops to see if they know of anyone locally that makes calibration boxes; they are fairly common. You can also check out United Speedometer Service at www.speedometershop.com. For later model Mustangs utilizing electronic vehicle speed sensors, electronic calibration boxes are available or a correction can be made in your custom tune. Ask us for details on either.

Manual Transmission Drive Gears
Teeth	Part Number	Color
6	E3ZZ-17285-B	Black
7	E3ZZ-17285-A	Yellow
8	F0ZZ-17285-A	Green

The drive gear in a T5/T45 transmission is pretty easily changed. Follow the steps below:

Drain and remove the transmission.
With the transmission in neutral, remove the shifter cover housing (four bolts) and shifter assembly.
Using a small punch, drive the roll pin down far enough to disengage the slider rail. Do not drive the pin all the way out of the shift gate.
Remove the bolts attaching the tail shaft housing to the main transmission case.
Slide the tail shaft housing, complete with shift gate, off the transmission. Be sure not to loose the detent ball and spring when the shift gate comes clear of the slider rail. Go ahead and remove the roll pin completely at this stage so that it can be driven in from the top side at reassembly.
Depress the spring clip that retains the drive gear and slide the drive gear off the output shaft.
Install the new drive gear and reassemble the tail shaft housing in the reverse order. A little petroleum grease will help hold the detent spring and ball in place while reassembling the tail shaft housing.