Tremec TKO Transmissions Part 4 – How To Choose The Right One
So you’ve decided that the benefits of the five-speed TKO are worth the expense and effort. Now the question is which one is the best fit for your car? 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 by torque, not horsepower. The maximum 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 capacity rating.
The second thing to consider is the rear axle ratio. The general rule, with exceptions, is anything with a two high or three low rear gear (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-line than a typical four-speed, and will help compensate for “highway gear” on the rear axle. Anything with an axle ratio of three upwards (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 more (numerically), your car doesn’t need as much “help” off the line as the numerically lower axle ratios do, and the TKO-600’s slightly higher overdrive with the 0.64 fifth gear will reduce the highway RPM a little more than the 0.68: 1 that the TKO-500 has. If you have an axle ratio in the middle of the three, (3.42, 3.50, 3.55, you can go for either one, and the correct choice will depend on the third and fourth factor.
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 provided by the tire must be taken into account. A 26-27 “tall tire is considered pretty normal for a muscle car. However, if you’re driving a 30” tall Mickey Thompson, the height of the tire will make a 3.73: 1 rear axle ratio work more. as a 3.25: 1 axle the ratio would be with a 26 “high rim!
The fourth thing to consider is your engine temperament, 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 jittery and doesn’t really start running clean until it hits 2800 RPM? Do you frequently drive long distances on the interstate at 75 MPH, or stick 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 lowers engine RPM too much. That’s when the TKO-600 with 0.82 overdrive may be the best option. Many people who participate in road racing prefer the TKO-600 with 0.82 overdrive, because it keeps its engine in its power band on a road circuit in fifth gear, and the 0.64: 1 ratio would lower its RPM too much.
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 10-spline Ford pattern input shaft or a 26-spline GM pattern input shaft. The TKO-600 is only available with the strongest 26 spline GM pattern. For the length of the input shaft, you basically need to know what size and year of motor you have, and how deep your steeple 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 need to consider your engine’s torque output, rear axle ratio, your tire size, your engine temperament, and your normal cruising speed. If you have a Ford, you will 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 are ready to choose the transmission that best suits your needs. For more information on TKO specs, options, and benefits of an overdrive, take a look at my other articles in this series.