Differential: Ford 9″ Detroit locker differential with standard ratio 3.89:1. This heavy duty, “no-spin,” limited-slip rear end was made by Detroit Automotive Products.

It is a semi-floating drop-out axle and had a GAWR up to 3,600 lb (1,630 kg).

4 3.83:1 Standard-Open

Mustang Axle Widths

67-71 Comet, Cougar, Mustang, Fairlane 59.25 inches

8935A is a WES U 3.50 28 spline open rear
4001A is a C8ZX A 3.89 31 spline open rear

On the Rear End Ratios

There were 18 ratios offered on Mustangs From 1965 til 1967: 9 conventional, and 9 locking. The codes are as follows.

A number designates a conventional while a letter indicates a locking rear end.

1.....3.00:1      A.....3.00:1
2.....2.83:1      C.....3.20:1
3.....3.20:1      D.....3.25:1
4.....3.25:1      E.....3.50:1
5.....3.50:1      F.....2.80:1
6.....2.80:1      G.....3.80:1
7.....3.80:1      H.....3.89:1
8.....3.89:1      I.....4.11:1
9.....4.11:1      L.....2.83:1
O.....2.79 (67 year only)


the 9” is a removable carrier type rear end

Axle Sizes  ( spline )

31 spline Axle diameter 1.291″ Bearing ID 1.562″  stock

If you tell someone you have a 9-inch Ford, automatically you’re a badass.” — Tom, Tom’s Differentials

No other rear has the same aftermarket following as the 9-inch Ford. The strength-to-weight ratio of the 9-inch Ford rearend is very hard to beat.” —Brydon Papenberg, Quick Performance

Ford’s 9-inch rearend was produced from 1957 through 1986. At one time or another, it was installed in nearly every Ford passenger car and truck. In racing and hot rodding today, you’ll find these rearends and their aftermarket descendents in just about everything—not just Fords.

But what makes the Ford “better”? Besides an inherently large ring-gear diameter (bigger than all competitive passenger-car performance rearends, except the massively heavy and relatively rare 9-inch Dana 60), the Ford’s biggest strength advantage over competitive rearends is its greater hypoid distance. Hypoid distance is the distance from the center of the ring gear to the center of the pinion gear. Given approximately equal ring-and-pinion diameters, the greater this distance (or gear offset), the higher the ring-and-pinion assembly’s inherent strength. That’s because offsetting the gears results in more gear-tooth surface contact at any given moment. The advantage increases at lower (high numerical) axle ratios because, compared to milder ratios, the low-ratio gearsets have relatively fewer pinion gear teeth in contact—meaning less teeth to spread and absorb the load.

Ironically, what became one of the Ford’s main strength advantages was not due to the design engineers’ desire to increase rearend strength. Back in 1957, Ford’s most powerful supercharged 312ci Thunderbird V8 put out 300 hp, with mainstream motors generally in the 200hp class at most. Instead, the large offset was a nod to appease the stylists who wanted a lower floorpan with less “center-hump” for the new Ford passenger-car body.

The 9-inch Ford Rearend is the Strongest Rearend Out There—It’s All Due to “Hypoid Distance”