Tremec TKO Transmissions Part 4 – How to choose the right one

So you’ve decided that the benefits offered by the TKO five-speed are worth the cost and effort. Now the question is which one is best for your car? Do you want the extra deep first gear in the TKO-500, or does the smaller gear spacing of the TKO-600 suit your combination better? Should you choose the overdrive ratio of 0.82 instead of the 0.64 in the TKO-600? If you have a Ford, what input shaft do you need?

The first determining factor when choosing a TKO is the power of your engine. Keep in mind that transmissions are rated for torque, not horsepower. Max horsepower doesn’t really matter, 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. While both TKOs can handle significantly more torque than they are rated for, you should still consider the rated torque.

The second thing to consider is your rear axle ratio. The general rule of thumb, with exceptions, anything with a rear gear in the high twos or low threes (2.73, 3.08, 3.23, 3.36, etc.) is a good candidate for the TKO-500. The low 3.27:1 first gear in the TKO-500 gives you much better off-line acceleration than a typical four-speed gearbox, helping to offset the “highway gear” in the rear axle. Anything with a shaft ratio in the high threes and above (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 “assistance” from the line as the numerically lower axle ratios, and the slightly higher overdrive of the TKO-600 with the 0.64 fifth gear lowers your highway RPM slightly more than the 0.68:1 that comes in the TKO-500. If you have an axle ratio in the middle of threes (3.42, 3.50, 3.55, you can go for both, and the right choice depends on the third and fourth factors.

The third factor to look at is your rear tire size. If you have a very tall rear tire, you need to consider the reduction in effective gear ratio that the tire offers. A tire that is 26-27″ tall is considered pretty normal for a muscle car. If you’re running 30″ Mickey Thompsons, your tire height will make a 3.73:1 rear axle ratio perform better than a 3 axle, 25:1 ratio would be with a 26″ high 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 content cruising at 2000rpm, or is your engine more tense and doesn’t really start to run clean until you hit 2800rpm? Do you often drive long distances on the highway at 75 MPH, or do you stick to secondary roads where you rarely exceed 60 MPH? It’s entirely possible to get too much overdrive and end up with a car that you can’t put in fifth gear unless you’re going at least 80 MPH, because at lower speeds the overdrive lowers your engine rpm too much. Then the TKO-600 with the 0.82 overdrive might be the best choice. Many people who compete in road racing prefer the TKO-600 with the 0.82 overdrive, as it keeps their engine in its powerband on a road course in fifth gear, and the 0.64:1 ratio should keep their revs make too low.

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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 splines and 26 splines, 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 with the stronger 26 spline GM cartridge. For the length of the input shaft, you basically need to know what engine size and year you have, and how deep your core is. Ford has used different core depths over the years, and there are more variations than I can cover here. Most Tremec distributors can help you determine which 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, 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 of manufacture, as well as the depth of the core. Once you understand how all these features work together, you’ll be ready to choose the transmission that’s best for you. For more information on TKO specs, options and the benefits of an overdrive, check out my other articles in this series.