So you’ve decided that the advantages offered by the five-speed TKO 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 mix? 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 for torque, not horsepower. The peak horsepower number doesn’t really matter 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 consider the torque capacity rating.

The second thing to consider is your rear axle ratio. The rule of thumb, with exceptions, anything with a rear gear in the top two or bottom three (2.73, 3.08, 3.23, 3.36, etc.) will be a good candidate for the TKO-500. The 3.27:1 low first gear on the TKO-500 will give you much better acceleration off the line than a typical four-speed, and will help compensate for “road gear” on the rear axle. Anything with an axle ratio between the tall three and above (3.73, 3.90, 4.10, 4.30, 4.56, etc.) would be more suitable 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 numerically lower axle ratios do, and the slightly higher overdrive of the TKO- 600 with the 0.64 fifth gear will drop the highway RPM a little more than the 0.68:1 on the TKO-500. If you have an axle ratio in the middle of the three (3.42, 3.50, 3.55), you can go with either, and the correct choice will depend on the third and fourth factors.

The third factor you need to consider is the size of the rear tires. If you have a very tall rear tire, the reduction in effective gear ratio that the tire provides must be taken into account. A 26-27″ tall tire is considered pretty normal for a muscle car. However, if you run Mickey Thompson 30″ tall, 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’s happy when cruising at 2000 RPM, or is your engine more jumpy and doesn’t really start running cleanly until it hits 2800 RPM? Do you frequently drive long distances on the interstate at 75 MPH, or do you stick to back roads where you rarely go over 60 MPH? It’s very possible to get too much overdrive and end up with a car that can’t get 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 powerband on a road course in fifth gear, and the 0.64:1 ratio would reduce engine speeds too much. RPM.

The last question to answer only applies to Ford owners. There are four different input shaft patterns available for the TKO on a Ford: 10-spline and 26-spline, short or long. The TKO-500 is available with either a Ford pattern 10 spline input shaft or a GM pattern 26 spline input shaft. The TKO-600 is only available in the stronger 26-spline GM pattern. For input shaft length, you basically need to know what size and year of engine you have, and how deep your bell housing 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 they have the above information.

When choosing a TKO that is right for your car, you should 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’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’re ready to choose the transmission that best suits your needs. For more information on TKO specs, options, and the benefits of an overdrive, check out my other articles in this series.