My mileage has dropped on my 1989 W350 when under load to about 12 mpg. I'd like to get back to 15-16 mpg when under load. I have installed a 47RH rebuilt trans and using toggle switches to activate the OD and locker. I have also installed an aftermarket intercooler. I tow in 3rd lockup at about 62 mph at 1900 rpm with the exhaust temp running below 1100. I've just had my pump set up and have good acceleration and horsepower. The mechanic said my pump was already advanced at 8 deg but I could go up to 12 deg. I recently replaced my injectors and just went a hair over stock, not even to the marine injector level. Do any of you have experience with the pump advance to 12 degrees? THANKS.

 

Did your mileage just magically drop, or did all of your listed mods make it worse?

 

My mileage has dropped off over time and that is what has prompted me to replace the injectors and have the injector pump adjusted. I just had the pump adjustment done yesterday. I did the injectors 3 months ago. Also my throttle cable replaced yesterday which helped throttle response and the truck was idling at 1100 rpm. Now idling at 800 rpm. I haven't checked the mileage yet but am asking about rotating the injector pump while still attached to fuel delivery lines to 12 degrees advance.