Tremec TR-3650 / TR-3550 / TR-4050

Tremec's rear-drive manual gearboxes below the T-56 — the TR-3550 and TR-3650 five-speeds and the TR-4050 beside them.

⚑ The TR-3550 is the old Borg-Warner T-5's bigger brother, and the TR-3650 replaced the T-45 in the Mustang. Tremec bought Borg-Warner's manual gearbox business, so a great deal of the material, the tooling and the part numbers carry across from names that no longer appear on anything.

They are built to be rebuilt, which is the practical difference from a modern automatic: parts are available, the procedures are public, and a worn synchroniser is a repair rather than a replacement.

The characteristic complaint is a notchy or blocked shift into second or third when cold, which is usually synchroniser wear or the wrong fluid rather than anything structural. Tremec specifies its own fluid and an ordinary gear oil produces exactly that symptom.

The TKO is the aftermarket version of the same gearbox and is what most modified cars carry, which is why searches return performance material for a standard-car question.

The T-56 and TR-6060 are the six-speeds above these and have their own page here.