I have a 03 gmc 2500hd I have the muffler removed I'm only getting 13 mpg driving easy I put on an edge juice w/attitude and no change in mileage the dash says it getting 18 it's not right I pencil out 13 any ideas

 

How are the injectors? Fuel filter? Even as simple as your tires and tire pressures?

 

I haven't checked my rates but I don't have smoke or haze my tires are good and I run about 70 psi filters were just changed

 

Do you still have your CAT on? It can be clogged and causing it not to run properly. How about air filter? I know these are some dumb questions, but a simple air filter change will help it run better..I personally know that. LOL

 

I've never noticed a cat under it and the air filter is new. The truck has 330 some thousand miles on it maybe its just to tired to get any miles might just be time for a Cummins swap

 

How new is your CP3? Check yoru fuel rail pressure on your CS / CTS monitor.