Things i should do first!

[RipCord]

I just purchased a 2005 gmc 3500 with a 6.6 lly crew cab long bed, the computer says its average fuel economy is 11.1. the truck has 102K miles on it.that kinda blows , whats the best performance parts i can put on it to make the motor come alive?. whats the best? banks, bullydog,hyper tech???

[JeepSJ]

First thing...gauges, especially EGT. Then add the power parts.

[Mitchagain]

2x....then flow-through muffler....then power adders

[JohnC]

First check the mileage by hand. There are many reasons why the DIC can be wrong. Also, confirm the odometer is correct.