For those who are moving to tires that are 35 inches or more, the rolling resistance, rotating mass, and the overall change in the gear ratio can affect the acceleration of the truck.
The bigger the tires you have, the larger the shift extension (here, the RPM usually drops between shifts), so the factory TCM calibration can have a “short shift” feel where the engine’s RPM drops too far and then the engine lugs and finds it hard to accelerate.
When you move the shift point later in the RPM range, the engine will stay in a higher rev range, leaving the engine’s output power exactly where it needs to be.
The other major adjustment within the TCM removes the major defuel felt on the 1 – 2 shift exchange at a wide open throttle. This can be better seen with a built transmission a higher horsepower application, and this does not mean that it cannot be noticed in stock transmission trucks, either, because it certainly can be seen.
The factory TCM commands the ECM to pull fuel before the 1 – 2 shifts so as to limit the torque that is being sent through the transmission during the shift. Once the transmission has been upgraded, it will be able to handle a higher torque load, so the big defuel will no longer be necessary as a safety net.
You can program the defuel accordingly so you can maintain a smooth forward motion from the truck, and it does not require the ECM and TCM programming so it can match for best results.
In six-speed applications, both the overdrive and the double overdrive gears can seem a bit hesitant to downshift, which would keep the RPM low, keeping the truck from accelerating on the highway like it normally should. Sometimes, you might have to apply more than 80 percent throttle so you can get the downshift to happen, usually, you do this when passing, and when the downshift happens, the ECM commands a lot of fuel, making the power output extreme and the truck jump, behaving erratically.