When I bought the truck it was a k3500 drw auto trans stock exhaust with 4.10 rearend.
I have changed the following that has increased fuel economy.
Change the TDC learn to -1.8. about +.25
Replaced the exhaust from the down tube back with 3.5 in mandrel bent exhaust. about +.5 mpg
Replaced the rear end with 3.73 and increased tire dia. to give equivalent of 3.43. about +1mpg
Removed bug shield. about +.1-.25
Use fuel conditioner. about +.25
Change to single rear wheels. about +1
Replace the auto with a NV4500 manual. about +1
Use all synthetic oils except the engine. about +.25
I have experimented with higher boost and water injection but have not been able to see any mpg gains with these the way I drive.
The above mileage gains may not quite add up but over all I think they contribute in about these proportions. I am hoping this summer to hit 16mpg pulling my camper with summer fuel since I have hit 15 with winter fuel.
'99 GMC K3500 6.5TD Crew Cab dually now SRW, Auto, now 5spd Yeah no more sissy slushbox
3.73 gears, 235/65 tires
Pull about 80% of time, mostly 4K lb hilo type camper