Mustangs with "low" rear-end ratios (3.91:1 or 4.30:1) often came equipped with a gear adapter (often called a reducer) to adjust the rotation of the speedometer cable. This adapter, combined with various speedometer cables, speedometer drive gears, speedometer driven gears, and tire size options, was used to ensure that the speedometer was accurate within specified tolerances.
Two types of adapters were used on 1968 - 1970 Mustangs: one type (part number C8ZZ-17294-A, marked C8ZF-17294-A) is straight, mounts to the driver's side of the transmission, and includes a .6944 gear ratio; the other (part number C9ZZ-17294-A, marked C8ZF-17294-B) has a 90-degree bend, mounts to the passenger side of the transmission, and includes a .7500 gear ratio. Here are pictures of both:
|C8ZZ-17294-A Adapter||C9ZZ-17294-A Adapter|
The applications for this adapter have been a source of confusion for a long time. Here's what I've been able to determine based on information gleaned from the 1975 edition of the Ford Master Parts catalog and owner-provided information:
Service Part Number
Engineering Part Number
|Manual||Not used||Not used||C7ZZ-17260-J, 77-1/16"1|
(before 31 January 1969)
(Picture 1, Picture 2)
(after 31 January 1969)
|Not used||Not used||C9ZZ-17260-A, 74"2|
- The 1975 Master Parts catalog lists only one speedometer cable for all 1968 Mustangs with a 4-speed manual transmission, with no distinction related to the rear-end gear ratio. I'm open to correction if a different cable was actually used.
- The 74" measurement appears in original engineering drawings and early editions of the Ford Master Parts Catalog. The 1975 "final" edition of the Master Parts Catalog identifies a later service part with a length of 66 1/2".
I have no explanation for why Ford stopped using an adapter after 31 January 1969 and then started using one again during the 1970 model year. Any insights into this situation would be welcome.
Note also that another straight reducer was used during the 1970 and 1971 model years for Fairlanes and Torinos equipped with 429 cubic inch engines and a 4-speed manual transmission. This adapter (identified by part number D0OZ-17294-A) is straight, mounts to the driver's side of the transmission, and includes a .7511 gear ratio.
Finally, here are pictures of installed adapters:
|Installed C8ZZ-17294-A Adapter||Installed C9ZZ-17294-A Adapter|
Thanks to Bill Upham of Mansfield Restoration Parts, Joe Sikora, Brian Conway, and Pat Whalen for the pictures used here. Additional thanks to John Thompson for providing me with the 1975 MPC pages to confirm the part numbers and applications listed here.