Converting a 1979-1993 Fox Body Mustang to a TREMEC TKX 5-Speed Transmission

The TREMEC TKX 5-speed has opened up a whole new world of driving enjoyment for classic and late-model cars and trucks. But many people wonder what is really involved in converting an automatic-equipped car to a TREMEC t-speed. We recently followed along with Knights Automotive in Oveido, Florida, as they made that conversion on a Fox Body Mustang.

The compact transmission case design of the TREMEC TKX allows for fitment in vehicles where a modern 6-speed manual transmission might not fit without major transmission tunnel modifications. The high torque handling capacity, high-rpm shifting ability and available gear ratios make it ideal to put behind almost any engine.

The 1979-1993 Ford Mustang is one of the most popular cars for car enthusiasts. They’ve proven to be highly adaptable for building in a variety of ways, from street performance to race cars. Ford built more than 2.5 million Fox Bodies, assuring there is still a plentiful supply of these Mustangs. But the majority of 1979-1993 Mustangs built were automatics, making factory manual transmission-equipped Fox Bodies more difficult to find. But converting an automatic-equipped Fox Mustang to a manual transmission is an easy, straightforward process.

Thanks to the design of the TKX, it easily fits in a Fox Mustang transmission tunnel without any modifications. The TKX actually takes up less space than the factory AOD automatic, and it weighs almost 200 pounds less than the AOD with torque converter. Capable of handling 600 lb-ft of torque, the TKX can easily handle a pushrod V8 or one of the modern modular or Coyote Ford OHC V8s. You can find all the details and specs on the TKX in our previous blog.

To show how easy converting a factory automatic Fox Mustang is, we found a suitable AOD-equipped test subject for the demonstration. This particular car is a factory SSP (Special Service Production) which were typically used as pursuit vehicles for state police, sheriffs and highway patrol units where a lighter, speedier and more nimble vehicle was needed over the typical police sedan.

ABOVE: Our subject Fox Body Mustang is a 1988 SSP (police car package) Mustang that didn’t see use as a marked car but likely as a supervisor or detective vehicle. Outside of the rearend gear ratio being changed to a lower gear, it is bone stock including a 5.0L V8. Knights Automotive in Oviedo, FL, completed the swap from an AOD automatic transmission to TREMEC TKX 5-speed transmission.
ABOVE: Most of the SSP Fox Mustangs that weren’t going to be marked cruisers ended up with the AOD automatic. Driving a Fox Body Mustang every day with the factory automatic is about as exciting as riding the escalator in a mall, so a change is definitely needed. There’s also the extra parasitic horsepower loss of the automatic that sucks up more of the 5.0L’s 225 horsepower versus a manual transmission.
Above: Because the TREMEC TKX 5-speed is longer than the AOD automatic, a shorter driveshaft is required. Instead of trying to find a used, stock manual transmission Fox Mustang driveshaft, a new aluminum driveshaft from Driveshaft Shop was used. Besides being lighter with less rotational mass than a factory steel driveshaft, it also has a much higher driveshaft speed/rpm which is beneficial with the Overdrive transmission and higher freeway speeds.
ABOVE: The Knights Automotive crew got everything disconnected so the AOD could drop right out with the use of transmission jack. The 5.0L has a few oil leaks from still having all its original gaskets, so the AOD was pretty oily-nasty.
ABOVE: The new TKX 5-speed is a few inches longer than the AOD, but a bit narrower at the widest parts. The TKX is almost 200 pounds lighter than the AOD with its torque converter. That is a lot of weight to be taking out of the car, with the typical correlation being 100 pounds reduction equals a tenth of a second off a car’s elapsed time in the ¼ mile.
ABOVE: With the AOD removed, work switched to the interior to remove the original center console and AOD shifter assembly.
ABOVE: Before Knights Automotive started installing new parts, this was the perfect time to install a new rear main seal to help cut down on some of the oil leakage from the engine. The pan gasket will be replaced later for obvious reasons too!
ABOVE: From the factory, automatic-equipped Fox Mustangs used a different transmission hump/tunnel than manual transmission cars, and notably did not have a hole for a manual-transmission shifter. For swapping in the TKX, an opening was made to fit the TKX’s shifter assembly.
ABOVE: To adapt the automatic transmission tunnel for the TKX manual transmission, Late Model Restoration Supply’s 1979-93 Mustang Manual Transmission Hump Kit (part no. LRS-7277A-K) was used. It comes with everything you need to the conversion the tunnel, including the factory lower shifter boot and all the necessary mounting hardware. The new stamped-steel section was riveted to the existing hump.
ABOVE: Here’s how things look from underneath before installing the TKX.
ABOVE: Replacing the AOD’s flexplate is a new McLeod aluminum flywheel, part no. 563100. The lighter weight of the aluminum flywheel versus a standard flywheel made of steel will allow the engine to rev faster and have less parasitic horsepower loss with the reduced rotational mass on the end of the crankshaft.
ABOVE: For the clutch, McLeod’s Street Extreme single-disc system was used, part no. 75307. It can handle up to 600 horsepower, and the disc’s friction material is great for street/strip configured vehicles. It uses a diaphragm-style pressure plate that requires a normal amount of effort for working the clutch.
ABOVE: The bellhousing is a standard TREMC unit for small-block Fords, part number TCEP8639. It comes with the necessary hardware to attach to the transmission, along with the pivot ball for the clutch fork.
ABOVE: Because the TKX is so much lighter than the old AOD, the transmission jack wasn’t even needed for installation.
ABOVE: The TKX looks right at home in the Fox Mustang’s transmission tunnel, not to mention way better than the automatic! And there’s more space now thanks to the overall narrower profile of the TKX versus the AOD was well. To hook the up the neutral safety and backup light switches into the car, wiring harness included with the TKX plug right in where the AOD’s original wiring connected.
ABOVE: Because the Mustang is going to see some time at the dragstrip, for extra safety a Stiffler’s driveshaft loop (part no. DSL-M02) was installed. The original transmission crossmember couldn’t be reused, so Knights Automotive also installed a new Stiffler’s tubular transmission crossmember, part no. TCB-M09.
ABOVE: The factory speedometer cable can be reused, but the speedo gear has to be changed. The gears are color-coded to correspond to certain rear gear ratios for accurate speedometer readings.
ABOVE: For this conversion, a factory 1979-1993 Mustang clutch pedal quadrant was used from Total Mustang Performance. The aftermarket isn’t making a new clutch pedal quadrant for the Fox Mustangs so a used one is the only option, but the quadrant was the same for four-cylinder and V8 Mustangs, so it doesn’t matter which engine the donor car giving up its quadrant had.
ABOVE: The 1979-1993 Fox Mustangs used a cable-operated clutch (as did the 1994-2004 Mustangs). Even though this SSP was originally an automatic, the factory hole for the clutch cable and adjuster was still there on the firewall and closed off with a rubber grommet. Since the clutch came from McLeod, McLeod’s clutch cable adjuster, part no. 1735 was also used.
ABOVE: To connect to the clutch pedal, McLeod’s clutch quadrant cable retainer, part no. 1734 was used.
ABOVE: Maximum Motorsports clutch cable, part no. MMCL-11, was used to connect the pedal to the clutch fork, part no. M-7515-A.
ABOVE: Part of the Late Model Restoration Supply conversion kit, part no. LRS-7277A-K, is a new lower boot to help seal the hole in the transmission hump. This limits heat, noise and debris from getting into the interior. It bolts right to the plate included with the kit.
ABOVE: Finishing things off inside the car is a new center console from Late Model Restoration Supply, part no. LRS-6104490S. While the original console could’ve been reused, now was the perfect time to give this Mustang some much need upgrades and modernization. For starters the new console has two cup holders where the original had none, and is already wired with a 12V lighter plug hookup and dual 5V 3.1 amp USB ports for charging modern devices. The built-in harness plugs right into the factory wiring.
ABOVE: Even with just a stock 5.0L under the hood, with the TKX the Mustang is much more fun to drive. The gearing of the TKX works well with the stock engine including overdrive on the highway, and the shifts are smooth as silk.