The AOD four-speed automatic...
The AOD four-speed automatic is the best choice for about 95 percent of street-driven vintage Mustangs. Several companies offer new AODs, like the Lentech Street Cruiser unit shown here, which includes a 12-inch non-lockup torque converter. Other sources for quality AODs include Performance Automatic and B&M. Of course, you can also get an AOD from a Fox Mustang donor car, but it's more of a crap shoot with a used transmission of unknown condition. It may need rebuilding before it can be used.
For more than a decade, installing an overdrive-automatic or manual transmission into a vintage Mustang has been a popular upgrade. It's more common these days, but some people still aren't convinced of the numerous benefits of such a conversion.
If you could only do one upgrade to your vintage Mustang that's a fairly nice driver and is reasonably complete, installing a transmission with an overdriven top gear is the single best thing you can do to improve the car's overall usefulness.
Even though we first owned a '66 fastback with an AOD more than 10 years ago, the novelty never wears off when cruising in an early Mustang on the freeway. Shift an AOD into Fourth gear or a T5 into Fifth, and engine speed is reduced by more than 30 percent while vehicle speed remains the same.
That said, there are numerous other benefits to an overdrive transmission, and reduced engine speed at swift highway velocities is only the beginning. Other major advantages include better fuel economy, less engine wear, quieter cruising on the highway, and in the instance of the typical T5, even better off-the-line acceleration due to a lower (numerically higher) First-gear ratio compared to four-speeds like the vintage Top Loader.
The T56 six-speed is much...
The T56 six-speed is much stronger than a T5, although at 115 pounds, it's at least 30 pounds heavier and considerably larger than a T5. Standard equipment in the '00 Cobra R, '03-'04 Cobra, and upcoming '07 Shelby GT500, a T56 can be installed in a vintage Mustang if you really want one. Various versions of the T56, available from D&D Performance, are strong enough to live behind, say, a 600hp Windsor. By the time you're done gathering up all the parts needed to do the swap, expect to spend around $4,000 for a complete T56 conversion, or about $1,200-$1,500 more than for a T5 swap.
We're covering swaps for both automatic and manual transmissions, as well as another popular swap, converting a car with an automatic to a manual. There are separate sections for each of these three possible avenues.
If an automatic to a five-speed manual is a swap you want to do, then it's easiest to go from an automatic straight to a T5 five-speed. Avoid doing the process twice by swapping to a four-speed, then realizing later that a five-speed is clearly the better option.
In any case, an overdrive transmission, either an automatic or a manual, is an upgrade you'll love more and more every time you drive your vintage Mustang. Having owned and driven numerous early Mustangs and other vintage Ford cars with either a T5 or an AOD, we can tell you the appeal of the conversion never wears off.
Since a majority of the vintage Mustangs produced came with either a C4, FMX, or C6 automatic, we'll begin with installing an overdrive automatic in their place. These days, it's nearly a bolt-in since much of the research has already been done.
To answer the most common question of "what overdrive trans should I use?" by far the best and most popular choice is the four-speed automatic-overdrive unit known as an AOD. The Ford AOD has at least as much aftermarket support as a C4, and there are numerous options for obtaining one.
New AODs with all sorts of modern internal updates are available from several sources, such as Lentech Automatics, Performance Automatic, and B&M Automotive, among others. New AODs also come with the correct torque converter for your application based on discussions with the companies that build the transmissions.