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Frequently Asked Questions
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Rear End Housings
Mustang housing widths are measured from the the flanges where the drum brake backing plates attach to the rear end housing ends, and not where the back of the wheel mates up to the "face" of the brake drum surfaces.
1. The '65-'66 Mustangs have a housing width of 52¼-inches
2. The Mustang saw an increase in track width starting in 1967. This increased the rear end housing measurement width by an additional 2-inches to 54¼-inches. This 54¼-inch housing width continued all the way through 1970.
3. In 1971, the rear end housings again increased in width, to 56-inches across.
4. In 1974, the Mustang's only rear end choice (with a "drop-out" style third member), was the 8-inch (no more 9's). The housing widths for these Mustangs dropped back to the original housing width of 52¼-inches. The only kicker here is that they are equipped with 4-lug axles and not with 5-lugs. To convert these housings to a 5-lug setup, you will have to have axles from a pre-'67 Mustang, or related Ford.
Some likely donor cars for rear ends:
1. 1960 T-Bird 9-inch rear end. It measures 52¾-inches.
2. 1957 Ford 9-inch rear end housing, 52¼-inches.
3. 1975-80 Granada or Monarch, (8" and 9" both were used) and Lincoln Versailles 9-inch disk brake rear end, 52¼-inches across. ( A word about the Versailles rear: although the housing width is same as 65-66 Mustang,
the actual width between the wheels is a bit wider due to longer axles used with the disc brakes. Also, those
disc brakes are VERY HEAVY and, IMHO,not really worth the effort and increase in unsprung weight.)
Rear End Ratio Selection
We are not talking economy here, just performance.
For a street car to be a strong performer, you need a torque multiplication ratio of at least 9:1. To figure this, multiply the transmission's first gear ratio by the rear end's ring & pinion ratio.
A C4 has a first gear ratio of 2.46:1.
An AOD has a first gear ratio of 2.40:1.
Example 1) 2.46 X 3.00 = 7.38:1 Torque multiplication ratio.
Example 2) 2.40 X 3.89 = 9.336 Torque multiplication ratio. (MUCH better!)
A Mustang with 26" tall tires, non-overdrive trans (1:1 in high gear), 3.00:1 rear end ratio, and travelling @ 60 MPH, will be turning roughly 2,326 RPM.
Given the same circumstances except for a AOD transmission change (.68:1 in overdrive), and 3.89:1 rear end gears, the same vehicle would be turning 2,051 RPM's! Rear end info provided by Steve Ainsworth - Thanks!