The TKO-500 is a 5 speed overdrive transmission introduced in 2004 to replace the TR-3550. Rated for 500 ft-lbs of torque, the TKO-500 is very stout. There are two models available. One comes with a 10-spline input shaft and one with a 26-spline input shaft. The output shaft has 31-splines for both models.
The TKO-600 is very similar to the 500, except even stronger. Rated for 600 ft-lbs of torque, the TKO-600 has different gearing than the TKO-500 and has two available over drive ratios. The TKO-600 only comes with a 26-spline input shaft. As such, when installing in a Foxbody or SN95 you will have to match this transmission with a new 26-spline clutch.
TKO-500 - 500 ft-lbs, 10-spline or 26-spline input shaft (this requires you to change the clutch to match)
Gear ratios: 1st-3.27, 2nd-1.97, 3rd-1.34, 4th-1.00, 5th-0.68
TKO-600 - 600 ft-lbs, 26-spline input shaft
Gear ratios: 1st-2.87, 2nd-1.89, 3rd-1.28, 4th-1.00 and an overdrive of either 0.82 or 0.64 (5th gear).
Installing in a SN95 (94-95):
If you are making over 500 ft-lbs of torque, then the answer to this question is easy. You need to go with the stronger TKO-600. However, if torque rating is not a factor then there is a general rule of thumb. As the gear ratios of each transmission are slightly different (the 500 has a shorter first gear), it is recommended to use a TKO-600 when the differential gears are 3.73 or lower (3.90, 4.10 etc). A TKO-500 is better suited for 3.55 and higher (3.27, 3.08 etc). If you will be using a TKO-500, it is recommended to opt for the 26-spline input shaft version. Even though this necessitates changing the clutch, overall it is stronger and less likely to strip. If a TKO-600 fits the bill, remember that there are two available overdrive ratios – either 0.82 or a much higher 0.64.
If you purchase the transmission new, it comes with a shifter from Tremec. All you need to add is your own handle. The shifter for both of these transmissions will line up in the factory location. Aftermarket performance shifters are available if you wish to shorten and improve shift feeling.
Despite the great power and torque capacity of these two transmissions, there are a couple of disadvantages. First and foremost, the TKO series transmissions are considerably heavier than a stock T5. Furthermore, they do NOT shift as smooth, especially at high RPM, and are noticeably noisier than the older T5. Notchy shifts can potentially be alleviated by replacing the stock Tremec shifter with an aftermarket unit from Steeda (#555-7456 - the Tri-Ax) or a Pro 5.0.