Back in the 1990s, 5.0 Mustang drag racers looking for a more durable manual transmission to replace their broken factory T-5 five-speeds began using Tremec’s 3550 five-speed gearbox. The 3550 was similar in that it used an aluminum case and an internal-rail shift mechanism, but it was much more robust than the T-5, and better suited to the rigors of drag racing. Later, Tremec introduced the TKO version of its gearbox with even greater durability; later still would come the TKO500 and TKO600, both with higher torque capacity ratings.

The TKO proved to be a terrific high-torque performance box, and classic muscle car owners then began trying to use this transmission in their cars to gain overdrive without sacrificing strength. However, the dimensions of the TKO gearbox posed fitment issues in certain vintage car applications. Fortunately, aftermarket specialists have developed kits to make swapping a TKO gearbox into a classic muscle or sports car fairly painless. Modern Driveline (MDL) is one of those specialists and offers several TKO transmission kits for specialized applications, like the Sunbeam Tiger, Corvette and other tight fit classic cars.

The popularity of the TKO as a swap gearbox motivated Tremec to return to the drawing board, yielding a new gearbox design, the TKX, which features more compact case dimensions, paired with improved internals. MDL has engineered the TKX to fit a broad selection of classic performance cars without modifications.

We worked with Miguel and Joseph Escamilla of M2 Performance in Los Angeles, who performed an MDL TKX swap on customer James White’s 1968 Chevelle hardtop, which packs LS2 power, a 4L60E overdrive automatic transmission, and a 12-bolt rear end. During this swap, the 4L60E was removed and replaced with the TKX.

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