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Tremec TKO Transmissions Part 4: How to Choose the Right One

So you’ve decided that the advantages offered by the TKO five-speed are worth the expense and effort. Now the question is what is the best combination for your car? Do you want the extra deep first gear on the TKO-500, or would the tighter gear spacing of the TKO-600 be a better match for your combination? Should I go for the 0.82 overdrive ratio on the TKO-600 instead of the 0.64? If you have a Ford, what input shaft do you need?

The first determining factor when choosing a TKO will be the power output of your engine. Keep in mind that transmissions are rated by torque, not horsepower. The peak horsepower number doesn’t really matter that much, it’s the 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 withstand considerably more torque than they are rated for, you still need to factor in the torque ability rating.

The second thing to consider is the ratio of your rear axle. The general rule, with exceptionsAnything with a rear gear in the high two or low three (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 on the TKO-500 will give you much better acceleration off the line than a typical four-speed, and will help offset “road gear” on the rear axle. Anything with an axle ratio between the high three and up (3.73, 3.90, 4.10, 4.30, 4.56, etc.) would be better suited for 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 TKO’s numerically lower axle ratios, and slightly higher overdrive, do. 600 with the 0.64 fifth gear will drop highway RPM a bit more than the 0.68:1 found on the TKO-500. If you have an axle ratio in the middle of all three (3.42, 3.50, 3.55), you can go for either one, with the right choice depending on the third and fourth factors.

The third factor to consider is the size of the rear tires. If you have a very tall rear tire, the reduction in effective gear ratio the tire provides must be taken into account. A 26-27″ tall tire is considered pretty normal for a muscle car. However, if you use a 30″ tall Mickey Thompson, your tire height will make a 3.73:1 rear axle ratio work. more like a 3.25:1 axle ratio would be with a 26″ tall tire!

The fourth thing to consider is the temperament of your engine, along with your driving habits. Do you have a relatively smooth engine that is happy when cruising at 2000 RPM, or is your engine more twitchy and doesn’t really start running clean until you hit 2800 RPM? Do you frequently drive long distances on the interstate at 75 MPH, or are you confined to back roads where you rarely go over 60 MPH? It’s quite possible to get too much overdrive and end up with a car that can’t put into fifth gear unless it’s running at least 80 MPH because at lower speeds overdrive reduces engine RPM too much. That’s when the TKO-600 with the 0.82 overdrive may be the best choice. Many road racers prefer the TKO-600 with 0.82 overdrive, because it keeps the engine in its power band on a road course in fifth gear, and the 0.64:1 ratio would reduce drag too much. RPM.

The last question to answer only applies to Ford owners. There are four different input shaft patterns available for TKO on a Ford: 10-spline and 26-spline, short or long. The TKO-500 is available with a 10 spline Ford pattern input shaft or a 26 spline GM pattern input shaft. The TKO-600 is only available in the stronger 26-groove GM pattern. For input shaft length, you basically need to know what size and year of motor you have, and how deep your hood is. Ford has used several different hood depths over the years, and there are more variations than I have room to go into here. Most Tremec dealers can help you determine which input shaft you need if you have the above information.

When choosing a TKO that’s right for your car, you should consider your engine’s torque output, your rear axle ratio, the size of your tires, your engine’s temperament, and your normal cruising speed. If you have a Ford, you’ll need to know the size and year of the engine, as well as the depth of the hood. Once you understand how all of these work together, you’ll be ready to choose the transmission that’s best for you. To learn more about TKO specs, options, and the benefits of an overdrive, check out my other articles in this series.

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