I have a 2005 300c 5.7, replaced rear end from the factory 2.82 to a 3.90 and now have issues. The mopar TCM (EGS53) that was in the car I believe is now smoked.
I used HP tuners to re-set the gear ratio and tire size and now the trans goes into limp mode with dtc p0614 (ecm/tcm incompatible). I have tried over and over again to try to resolve this. I then proceeded to replace the mopar TCM (EGS53) with the factory TCM (EGS52) and now have a 4th gear ratio error and goes into limp mode. The p0614 dtc is now gone but I can't set the gear ratio to 3.90 using HP tuners.
The production date on the car is 11/04 if that is of any use.