So you have decided the advantages that the TKO five speed provides are worth the expense and effort. Now, the question is which one is the best match for your car? Do you want the extra deep first gear in the TKO-500, or would the closer gear spacing of the TKO-600 be a better match for your combination? Should you opt for the 0.82 overdrive ratio in the TKO-600 instead of the 0.64? If you have a Ford, which input shaft do you need?
The first determining factor when choosing a TKO is going to be your engine’s power output. Keep in mind that transmissions are rated for torque, not horsepower. The peak horsepower number doesn’t really matter much, it is torque that breaks things! The TKO-500 is rated for 500 lb.-ft. of torque, and the TKO-600 is rated at 600 lb.-ft. Even though both TKOs will stand up to considerably more torque than they are rated for, you still need to keep the torque capacity rating in mind.
The second thing you need to consider is your rear axle ratio. The general rule of thumb, with exceptions, is anything with a rear gear in the high twos or low threes (2.73, 3.08, 3.23, 3.36, etc.) will be a good candidate for the TKO-500. The low 3.27:1 first gear in the TKO-500 will give you much better off the line acceleration than a typical four speed, and help compensate for the “highway gear” in the rear axle. Anything with an axle ratio in the high threes and up (3.73, 3.90, 4.10, 4.30, 4.56, etc.) would be better suited to the TKO-600. With a rear axle ratio of 3.73:1 or higher (numerically), your car doesn’t need as much “help” off the line as the numerically lower axle ratios do, and the slightly taller overdrive of the TKO-600 with the 0.64 fifth gear will lower your highway RPM a little bit more than the 0.68:1 that is in the TKO-500. If you have an axle ratio in the mid-threes, (3.42, 3.50, 3.55 you could go with either one, and the right choice will depend on the third and fourth factors.
The third factor you need to look at is your rear tire size. If you have a really tall rear tire, the reduction in effective gear ratio that the tire provides needs to be taken into account. A tire that is 26-27″ tall is considered fairly normal for a muscle car. If you are running 30″ tall Mickey Thompsons, however, your tire height will make a 3.73:1 rear axle ratio perform more like a 3.25:1 axle ratio would with a 26″ tall tire!
The fourth thing to consider is your engine’s temperament, along with your driving habits. Do you have a relatively mild engine that is happy when cruising at 2000 RPM, or is your engine more high-strung and doesn’t really start to run cleanly until you hit 2800 RPM? Do you frequently drive long distances on the interstate at 75 MPH, or do you stick to secondary roads where you rarely go more than 60 MPH? It is very possible to get too much overdrive, and end up with a car that you can’t put into fifth gear unless you are running at least 80 MPH because at lower speeds the overdrive drops your engine RPM too much. That is when the TKO-600 with the 0.82 overdrive may be the best choice. Many people who participate in road racing prefer the TKO-600 with the 0.82 overdrive, because it keeps their engine in its powerband on a road course in fifth gear, and the 0.64:1 ratio would drop their RPM too low.
The last question to answer only applies to Ford owners. There are four different input shaft patterns available for the TKO in a Ford: 10 spline and 26 spline, short or long. The TKO-500 is available with either a 10-spline Ford pattern input shaft or a 26-spline GM pattern input shaft. The TKO-600 is only available with the stronger 26-spline GM pattern. For the input shaft length, basically you need to know what size and year engine you have, and what depth your bellhousing is. Ford has used several different bellhousing depths over the years, and there are more variations than I have room to get into here. Most Tremec distributors can help you determine what input shaft you need if you have the above information.
When choosing a TKO that is right for your car, you need to consider your engine’s torque output, your rear axle ratio, your tire size, your engine’s temperament, and your normal cruising speed. If you have a Ford, you will need to know the engine size and year, as well as the bellhousing depth. Once you understand how all these work together, you are ready to choose the transmission that suits your needs the best. For more information on TKO specifications, options, and the benefits of an overdrive, take a look at my other articles in this series.