For the longest time, if you ran a Ford small-block or Windsor engine and wanted to run an automatic, your transmission of choice would likely be a C4. For heavier duty applications you'd likely opt for a C6. As with everything, time and technology move on and today many overdrive automatics are available, mostly computer-controlled. With internals loosely based on the FMX, the four-speed overdrive AOD which was offered from 1981-93 is the only non-computer controlled overdrive Ford transmission that will work with a V-8.

Ford first introduced an automatic transmission in the Fifties (discounting the Hydramatic offered in 1949-54 Lincoln Continentals), with the three-speed Ford-O-Matic, in service from 1951 to 1968. With a separate bellhousing, this cast iron cased trans evolved into the FMX, which lasted until 1981. However, unless you're restoring your Ford, you'll likely want to upgrade from the Ford-O-Matic, which was also known as the Merc-O-Matic, or Cruise-O-Matic, though we've also heard the latter used to describe an FMX trans.

Used in Ford, Lincoln and Mercury passenger cars and Ford light trucks until 1979, the FMX is easily recognizable by its cast-iron main case, which differentiates it from other Ford three-speed automatics, with separate aluminum bellhousing and extension housing. While the FMX can handle plenty of torque-indeed it was used in trucks into the Eighties-it's not as well catered to by the performance aftermarket as the C4 and C6 are, and it's heavy, thanks to that iron casing. Rated as a medium duty transmission, the final drive ratio is 1:1.

Ford's most adaptable auto transmission for years has been the C4, the light-duty three-speed introduced in 1964 and offered behind four-, six- and small eight-cylinder engines, namely the 260, 289, 302 and 351W motors. You'll find them in Galaxie, Fairlane, and some Falcon models. The C4's removable bellhousing means it's long been a favorite automatic to bolt behind a Flathead too, thanks to companies like Flat-O-Matic offering bellhousing adaptors. With an aluminum case, the C4 is light in weight and efficient, comparable to the GM Powerglide as one of the least power-robbing automatic transmissions. There are two variants, however. Those in passenger cars have the dipstick in the case, while van, truck, and some fullsize car versions have the dipstick in the trans pan. Larger bellhousings accommodate a 12-inch converter; an 11-inch converter is found in the versions with the smaller bellhousing. They are identical internally. A lockup converter version called the C5. which uses different valve body programming, ran from 1982-86, when C4 production ceased. It too has the larger bellhousing and converter, and can be found in LTD II, Ranger, Fairmont, Cougar, T-Bird, and Mustang models.

Moving up to Ford's longtime heavy-duty transmission, the C6, which can handle large amounts of torque, and came from the factory bolted to the rear of 351W, 351C, 428, 429 motors and even the 7.3 liter diesel, Ford moved to a one-piece casing with integral bellhousing. Produced from 1966-89, again with a final drive ratio of 1:1, the C6 is to the C4 what the GM TH400 is to the TH350. The other gear ratios are: 2.46 First gear and 1.46 Second gear. Available in one of three bellhousing bolt patterns, depending on which series engine it was coupled to, the C6 was the transmission of choice for Ford drag racers for a long time, until strong race-prepared C4s became available.

Ford's first overdrive automatic transmission, and indeed the first for any American automaker, was the AOD (Automatic Overdrive) 4-speed, introduced in 1980 and available until the early Nineties when the AODE began to replace it, starting with the new modular V-8. With mechanical lockup, it can be found behind many Ford motors from the 3.8 liter V-6 to 351ci V-8s. With an overdrive ratio of 0.67:1, gas consumption and engine cruising speed were greatly reduced over its predecessors, though obviously rearend ratio also affects this. The other ratios are First: 2.40:1, Second: 1.47:1, Third:1:1. According to Phoenix Transmission Products' Greg Ducato, "The AOD is a good swap choice from a C4, C6, or FMX transmission and shares many common dimensions with them. The AOD also uses the small-block bellhousing pattern which fits 250 and 300 six-cylinder engines, as well as 289, 302, 351 Windsor and 351 Cleveland."

The AOD transmission and torque converter are a matched pair, as the AOD uses a hollow two-piece input shaft for mechanical lockup, and as such requires a specific torque converter.

By 1994 the AOD had been superseded by the AODE (the E denoting "electronic"), sharing most internal parts except the input shaft, and electronically controlled by a computer, doing away with the TV cable. Advantages are that the shift points and firmness, as well as the lock-up point can be programmed. Incidentally, a wide ratio version of the AODE, using a 2.84:1 First gear is also available, known as the 4R70W.

For extreme tow or race duties, there's also the E40D, which is huge, essentially a C6 with an overdrive added on, but it can handle serious power, and was used in Lightning pickups.

We're not sure how many of you might contemplate dropping a late-model Mustang (2002 onwards) drivetrain in your project, but the 5R55S transmission they use is a five-speed, with four forward gears and an overdrive. First gear is 3.25:1, low enough to provide rapid acceleration! Don't confuse them with the 5R55N or 5R55W though, which are also five-speeds but came behind four- and six-cylinder engines despite the similar numeric designation!

Before you rush to buy and bolt an overdrive behind your engine though, take some time to calculate whether your engine/rearend/tire combo will support one. With a normally aspirated motor and maybe a mild cam, you should be looking at a 70 mph cruising speed of around 2,000-2,200 rpm. If you are running 28-inch diameter tires and a 2.56 rear gear, you're not going to be able to use Overdrive. Of course you may want to switch to a shorter gear (numerically higher number) to take advantage of the Overdrive and quicker acceleration in the other gears, but now your bill is mounting up! What we're saying is you need to know your objective when selecting a transmission, and that your engine needs to be within its operating range for fuel efficiency. For more on this refer to our "Hitting the Sweet Spot" article in our February 2009 issue or in the tech section at www.rodandcustommagazine.com.

Okay, now you're armed with some history and model designations, which Ford automatic transmission is right for your project? How much power do you need it to handle? Do you need an overdrive? Will the trans of your choice fit in your car? To help with that decision we'll look at some strengths and weaknesses of each model, physical sizes and lengths, and power ratings, with the help of some aftermarket transmission suppliers. First though, let's compare weights and lengths. The following are all without fluids or torque converter:

The C6 has always been regarded as a heavy transmission, but as can be seen, the AOD outweighs it by another 10lbs. However, the advantages of the overdrive gear make up for this.