To clear up any confusion about the "World Class" nomenclature, all T5s installed in Mustangs in 1985 and after are considered "World Class" units. Borg Warner upgraded the T5 to "World Class" status by adding tapered roller-cluster bearings; caged needle bearings under First, Second, and Third gears; double synchros and First and Second gears; and friction material bonded to First through Fourth-gear blocker rings. However, these '85-'89 T5s with their 3.35 First gears were only rated with a 265 lb-ft torque capacity-so among all "World Class" T5s, some are better than others. If the T5 in your '89 or older Fox 5.0 needs to be replaced (or it can be rebuilt), you'll want to upgrade to the following stronger innards or a '90 or newer unit.
After numerous instances of breakage (mainly Third gear) on the '85-'89 units, Ford and Borg Warner made a number of improvements for 1990. These included a coarse-pitch Second and Third gearset (fewer yet stronger teeth) and specifying 4615 steel for Third gear and the cluster, and 4620 steel for Second gear. This increased the torque capacity rating to 300 lb-ft. If you're having an '85-'89 T5 rebuilt, you can install these stronger pieces available as a kit from D&D Performance.
The production T5 was improved again for the '93 Cobra by replacing the 15-needle bearings found in the input gear "pocket" with a tapered roller bearing, which reduced mainshaft deflection and improved gear-tooth contact. This Cobra-only T5 had an increased torque-capacity rating of 310 lb-ft. This setup was also used in '94-'95 Cobras, but not added to T5s used in regular '93-'95 GTs. And just so you know, all T5s use Dexron III ATF for lubrication.
What does all this mean to those who might need another transmission? Our recommendations are as follows: If you're going to have your existing transmission rebuilt, be sure you add the '90-and-newer type upgrade kit available from D&D Performance (PN AK1). If you're going to buy a used T5 from another car, we'd steer towards one from a '90 or newer 5.0, preferably with as little mileage as possible.
If you want to upgrade to a brand-new T5, the best bet would be to go with the FRPP aftermarket heavy-duty unit available from D&D, Ford Racing Performance Parts, or any other FRPP dealer such as Dallas Mustang. It carries PN M-7003-Z.
Tremec T45 And TR-3650
The 4.6-liter modular V-8 appeared in the '96, and along with it a new transmission known as the T45. Both GT and Cobra models use the T45, which has an integral clutch cover (bolts on from inside the housing). The T45 will not work with pushrod 5.0 (or 351W) engines due to its 4.6-specific bellhousing bolt pattern. While it has an impressive 375 lb-ft torque-capacity rating, in some circles it's known to have less-than-bulletproof internal shift components such as forks and Z-links. However, since its production began, improvements have been continuous. Like the T5, the T45 uses Dexron III ATF.
In the middle of 2000 production, a new transmission was introduced for the Mustang. Dubbed the TR-3650, it's a clean-slate design from the ground up, totally separate from the T45 it replaces. We spoke with the folks at D&D Performance and this unit is still so new that they don't have in-depth info on it yet, but will probably know more later this year. The best way to describe a TR-3650 at this point is that it's an amalgamation of the T45, the TR3550 and the T56 six-speed. Since the switch occurred midyear during the 2000 GT production run (recall there were no 2000 Cobras produced other than the Cobra R which was equipped with a T56) those who own a 2000 GT might have a car with a T45 or a TR-3650. It's easy to tell the difference: If your car's transmission has the drain plug on the bottom of the case, then it's a TR-3650. If it's on the side, then it's a T45.