The real joy of owning a muscle car is in driving it. Muscle car owners who want to get the most out of their cars often drive to distant car shows or other events, but without an overdrive transmission a long trip on the highway becomes tedious. If the car has a performance rear axle ratio it is even worse. I love the sound of a V8 at speed as much as any guy out there, but a steady 3600 RPM for three hours solid with loud exhaust gets tiring. Most of us have become accustomed to being able to hold a conversation and/or hear the radio on long drives, and we would like to be able to pass by at least a couple of gas stations without stopping to fill up! An overdrive transmission solves those problems, and provides many other benefits as well.
The Tremec TKO is an overdrive manual five speed that is ideal for retrofitting into the classic muscle cars of the sixties and seventies. With your choice of a 0.82:1, 0.68:1, or 0.64:1 overdrive ratio depending on the model, you can really bring those revs down to a manageable number. A lower engine RPM on the highway provides several advantages.
First of all, as already mentioned, is the noise level. If you have a 3.73 axle ratio with a 235/60-15 tire (26.1″ tall), 75 MPH equals 3600 RPM with a non-overdrive four speed. The TKO-600 with a 0.64:1 fifth gear (the most popular version) will bring that 3600 RPM down to a leisurely 2300 RPM at the same 75 MPH. The sound level at 2300 RPM is a whole lot easier on the ears after a couple or three hours than 3600 RPM would be!
Another benefit of the lower RPM that I already touched on is increased fuel economy. All other things being equal, the engine will obviously use less fuel at 2300 RPM than it will at 3600 RPM, but with a muscle car engine that has been built for performance there is another factor that comes into play. At 3600 RPM, some 4-barrel carburetors may have the secondaries partially open. If you have a steeper axle ratio, it is even more likely that the secondaries are open at highway speeds. With a 4.10 rear end gear, that 75 MPH number I am using results in almost 4000 RPM with a non-overdrive transmission. At that point, many carburetors are going to have the secondary barrels dumping fuel. Reducing the RPM needed to cruise on the highway will reduce the throttle angle, closing the secondaries and making a huge difference in fuel economy. I have actually seen fuel mileage on the highway be doubled by installing a TKO, due to being able to cruise without having the secondaries open!
Engines wear faster at higher RPM. In simplified terms, every engine has a lifespan that consists of a certain number of revolutions, barring abuse, neglect, or catastrophic failure. If you use fewer of those revolutions every time you drive 200 miles to a car show, your $5,000, $10,000, or even $20,000 engine will last longer. So will everything attached to it that turns with the engine. Your alternator, water pump, mechanical fuel pump, power steering pump, and AC compressor will all be turning fewer RPM on the highway, and their life will be increased.
High performance engines produce more heat than normal passenger car engines do. Many muscle cars have heat problems due to high RPM on the highway. Reducing the RPM with an overdrive can reduce engine temperature and prevent overheating on long trips.
The intangible advantage to all the above is that overdrive will enable you to drive your muscle car more. You will be able and more likely to drive it to further-away car shows, or even on vacation! Overdrive allows you to enjoy your car more by driving it to places that you wouldn’t consider going with your 4-speed (or 3-speed automatic) spinning the engine at 3600 RPM.
The TKO is plenty strong for most muscle cars. It is rated for either 500 or 600 lb.-ft. continuous input, and those numbers are very conservative. There are a large number of cars running much more than the rated torque with the TKO, and failures are rare. Although I can’t officially condone using the TKO in an application that exceeds the torque capacity rating, I can tell you from experience that many folks have done so and have not had any problems, even under race conditions.
The Tremec TKO will decrease the noise level on the highway, increase your fuel mileage, increase the life of your engine and everything attached to it, could reduce or eliminate temperature issues, and allow you to drive your car to places that you wouldn’t with an old four speed. It is stronger than the old Muncies, T-10s, Toploaders, and A-833s. Versions are available that will bolt directly up to a common GM or Ford 4-speed bellhousing, and custom Mopar bellhousings are available to adapt a TKO to a Mopar small block, big block, or late model Hemi. All in all, it is an ideal transmission for a muscle car that gets driven on the highway. For more detailed information on TKO specifications and options, take a look at my other articles in this series.