I have a set of 3.73 gears I want to put in the CLK, but if you change the gear ratio you need to reprogram the Transmission Control Unit (TCU) so it knows how to perform its speed calculations. I could have a tuner modify my stock
TCU, but I decided to take this opportunity to replace the factory TCU with a TCM2800 aftermarket controller by Powertrain Control Solutions. I got the controller from Sound German Automotive in Bellevue, Washington. Russell at SGA is a 722.6/NAG1 transmission guru who knows everything you could imagine about these transmissions.