Tremec TKO Transmissions Part 3: The Ideal Overdrive Transmission for a Muscle Car
The real joy of owning a muscle car is in driving it. Power car owners who want to get the most out of their cars often drive to distant auto shows or other events, but without an overdrive transmission, a long road trip becomes tedious. If the car has a performance rear axle ratio, it’s even worse. I love the sound of a revving V8 as much as anyone else, but a steady 3600 RPM for three straight hours with a loud exhaust is tiring. Most of us have grown accustomed to being able to carry on a conversation and/or listen to the radio on long journeys, and we wish we could pass at least a couple of gas stations without stopping to refuel! An overdrive transmission solves those problems and provides many other benefits as well.
The Tremec TKO is an overdrive five-speed manual transmission that is ideal for retrofitting to classic muscle cars from the 1960s and 1970s. With your choice of a 0.82:1, 0.68:1, or 0.64:1 overdrive ratio depending on the model, you can really get those revs down to a manageable number. Lower engine RPM on the highway offers several advantages.
First of all, as already mentioned, there is the noise level. If you have a 3.73 axle ratio with a 235/60-15 (26.1″ tall) tire, 75 MPH equals 3600 RPM with four speeds without overdrive. The TKO-600 with a 0.64:1 fifth gear (the version most popular) will drop that 3600 RPM down to a quiet 2300 RPM at the same 75 MPH.
Another benefit of lower RPMs that I already mentioned is better fuel economy. All things being equal, the engine will obviously use less fuel at 2300 RPM than it will at 3600 RPM, but with a high performance 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 secondaries partially open. If you have a steeper axle ratio, the secondaries are even more likely to be open at highway speeds. With a 4.10 rear gear, that 75 MPH number I’m running works out to almost 4000 RPM with a non-overdrive transmission. At that point, many carburetors will have the secondary barrels dumping fuel. Lowering the RPM needed to cruise on the highway will lower the throttle angle, closing up the secondaries and making a big difference in fuel economy. In fact, I’ve seen highway gas mileage double when installing a TKO, due to being able to cruise without having the secondaries open!
Engines wear faster at higher RPMs. In simplified terms, every motor has a useful life consisting of a certain number of revolutions, barring abuse, neglect, or catastrophic failure. If you rev less than those every time you drive 200 miles to a car show, your $5,000, $10,000, or even $20,000 engine will last longer. This will be everything that is attached to it and that rotates with the engine. Your alternator, water pump, mechanical fuel pump, power steering pump, and AC compressor will turn less RPM on the highway and increase lifespan.
High performance engines produce more heat than normal passenger car engines. Many muscle cars have heat issues due to high RPM on the highway. Lowering the RPM with an overdrive can reduce engine temperatures and prevent overheating on long drives.
The intangible advantage of all of the above is that overdrive will allow you to drive your muscle car more. You will be able to and will be more likely to drive it to car shows further afield, or even on vacation! Overdrive allows you to enjoy your car more by driving it places you wouldn’t consider going with your 4-speed engine (or 3-speed automatic) spinning the engine at 3600 RPM.
The TKO is pretty strong for most muscle cars. It is rated for 500 or 600 lb.-ft. continuous input, and those numbers are very conservative. There are a large number of cars that run much more than rated torque with the TKO, and failures are rare. While I officially cannot condone the use of the TKO in an application that exceeds the torque rating, I can tell you from experience that many people have done it and had no problem, even under race conditions.
The Tremec TKO will decrease the noise level on the road, increase fuel consumption, increase the life of your engine and everything related to it, could reduce or eliminate temperature problems and allow you to drive your car to places where you don’t I would. With an old four speed. It is stronger than the old Muncies, T-10, Toploader and A-833. Versions are available that will bolt directly to a stock GM or Ford 4-speed hood, and custom Mopar hoods are available to adapt a TKO to a Mopar small block, big block, or late-model Hemi. All in all, it’s an ideal transmission for a highway-driving muscle car. For more detailed information on TKO specs and options, please see my other articles in this series.