[Phantom]

Only driving 8 miles will kill the MPG since the car will not use the some of the sensors till it has warmed up. Also the transmission has two tables one for cold and one for warm in the cold setting it will hold longer before shifting or locking the Torque Converter.

I'm not familiar with the cars ECM but look around for a tuning program like HPTuners (note they do not have support for your car currently but you could contact them). You would not even have to but the programmer as long as you find some one local that has it.

OR you could try finding someone with a TECH2 (dealer often or a great shop) and see if they can scan and lower the 4th shift and TC lock/unlock about 3 MPH.

Good luck.