The Mustang's First Automatic Transmission

The Mustang's First Automatic Transmission

Last Updated August 8, 2023 | Meghan Drummond

Ford’s C4 Transmission

Though the Mustang’s manual transmissions have traditionally been more popular, offering an automatic transmission was one of the many reasons that Mustangs became America’s favorite pony car as soon as Ford introduced them. Automatic transmissions were still relatively new, but the Mustang’s launch happened to coincide with some of the first major improvements in automatic transmissions.

An old ‘60s advertisement for the SelectShift transmissionFord SelectShift

The first automatic transmission to be offered in the Ford Mustang was the C4, which replaced Ford’s previous Ford-O-Matic transmission. At the time, the C4 was referred to as the dual-range Cruise-O-Matic, but many enthusiasts refer to it as the green dot transmission. This nickname stems from the two dots on the gear selector. The small dot was for keeping the transmission in second gear for icy conditions while the large dot was for normal driving. This confusing dot system was eventually deserted and replaced with the much easier to understand shifter pattern we’re used to seeing today. It was also redubbed the SelectShift, or occasionally the SelectShift Cruise-O-Matic.

The C4 was a longitudinal three-speed automatic transmission that improved on the design of the Ford-O-Matic by using a Simpson planetary gearset, which reduced the weight significantly. This transmission also used aluminum alloy components as opposed to the cast iron of the previous generation which reduced the weight further. All told, the C4 cut nearly 118 pounds from the weight of the Ford-O-Matic. Dry (without fluid or a torque converter), the C4 weighed in at a nimble 110 pounds.

C4 Transmission Gear Ratios

Unlike many other transmissions, the C4 only had one set of gear ratios through all the years of its production.

C4 Gear Ratios
First 2.46:1
Second 1.46:1
Third 1.00:1
Reverse 2.18:1

Changes to the C4 by Year

The C-4 was a remarkably long-lasting transmission and was in use until 1981, at which point it was replaced with the C5, which was essentially a C4 transmission with a locking torque converter.

One of the most important changes to happen to the C4 was the increase from 24 to 26 splines on the input shaft, which increased the durability of the transmission.

Where to look for transmission plate
Plate Locations for Ford Automatic Transmissions

The C4 was primarily used in V6 and smaller V8 engines and had a relatively low torque capacity. It is still popular with hotrodders, restorers, and modifiers due to how easy it is to modify and work on this transmission. Though the C4 was a very long lasting transmission there were several modifications made every year that makes some versions incompatible with parts from previous years.

C4 Specifications by Year
YearsTransmission Code Input ShaftValve BodyShifting Pattern
1965-1966 C4 Transmission 6 0.788 inches with 24 splines on both ends C4 or C5 casting number P R N D2 D1 L
1967-1969 C4 Transmission W 0.788 inches with 24 splines on both ends C7 Casting Number P R N D 2 1
1970 C4 Transmission W 0.839 inch with 26 splines on both ends D0-D7 Casting Number P R N D 2 1
1971-1980 C4 Transmission W 0.839 inch with 26 splines at the torque converter and 24 splines at the clutch hub D0-D7 Casting Number P R N D 2 1

Modifying C4 Transmissions

Though the C4 is an older transmission style, its simplicity lends it to rebuilds, and it’s a popular choice with many for this reason. It’s also one of the most dependable transmissions. It's noted that the post-1970 C4s are significantly more durable thanks to the additional splines that were added.

Two input shafts side by side
24-Spline (Left) vs 26-Spline (Right) Input Shaft

The stock C4 was frequently matched with a Windsor that could make 333-lb-ft of torque. If you elect to rebuild a vintage transmission but pair it with an improved engine, that’s entirely possible with a few modifications.

Improve Torque Converter

One of the most popular modifications is to outfit the C4 transmission with an aftermarket torque converter. One major advantage of an aftermarket torque converter is the ability to set your own stall speed. By ensuring that your stall speed and the RPM range where your engine finds its peak power band match up you guarantee an improved experience that would be suited for a drag racing application as well as a cruising one, depending on what style of torque converter you purchased.

Replace Seals

A leaky transmission isn’t going to be a strong transmission. It’s recommended that you trade out seals and gaskets for newer ones as you work on your transmission. Most rebuild kits will have a full set of improved caskets. A rebuild kit may also have additional parts that will allow for an improved build.

Improve or Add Clutches

In some rebuild kits, you’ll see racing clutches, new friction plates, and new steel clutches. This should improve the “bite” of the transmission. Some hot rodders have also had great success by adding clutch plates to their c-4 transmissions.

New C4 Transmissions

If you’re looking at buying a vintage transmission and rebuilding it, the cost can add up quickly. On the other hand, a lot of companies have started producing C4 transmissions that are already upgraded and ready to go. Typically, modern C4s have a much more robust capability. These C4 transmissions keep the “easy to use” and “easy to install” aspects that people liked about the C4, but they also improve the quality of several key pieces in order to make the transmission capable of handling increased output and high performance.

There are a lot of performance rated C4s that are readily available if you’re replacing the transmission on a vehicle include a lot of valuable upgrades like a hardened input shaft or improved servo. The amount of horsepower that they can handle ranges from 450 to 1,000 hp.

Sources: How to Rebuild and Modify Ford C4 and C6 Automatic Transmissions, CarTech | Ford C4 Identification, It Still Runs | Ford Automatic Transmissions, An Anecdotal History Image Credit: Ford | Ford Dodge Transmissions

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