I am getting and have always gotten 10-11 mpg Hwy on my 1998 2500 4x4 with the 454 Vortec and the 4L80E A/T. NO MATTER WHAT I HAVE DONE OR HOW I DRIVE mileage does not improve!

I have added true duals/No cats and with 3" pipes w/ flowmaster big block mufflers.
K & N Air filter. Helix Spacer. Synthetic Oil. New plugs , Hi-po Wires. Even tried Acetone in the Fuel. Nothing seems to help at all.

On start up I always hear a rappping noise that goes away after about 1 minute.

 

i would guess, without hearing it, that the rapping noise is either the injectors, or more likely the lifters building up...once it gets some ehat and the oil has thinned out a bit, they hydraulic lifters build up properly.

as for the fuel mileage, i dont get much better in my trucks, with smaller engines.

 

try replacing the cap and rotor. I get an average 12-13mpg with my 98 gmc 454, 3.73 gears, since the day i bought it (Mar 98).
good luck!

 

i dont know if i would even fret about 1mpg...