My first cummins was in a 99 dually five speed. got around 17 mpg, sold and got a 94 2wd bone stock with around 200,000. It got consistent 21 mpg. In 03 got a new 4 door 6 speed. 22 to 23 back and forth to work, driving easy. It got stolen and burned by some little bastard kids. I found a nice 94 4X4 auto. I got 20 mpg on the way home form Texas, but around home back and forth to work 17. I had the timing turned to 16 put a shorter belt to bypass fan and pulley and now it is 16 mpg on last fillup (driving like a little blue haired lady) rpm's under 2,000. Any tips or sugestions would be great.

Is the auto that much worse or is it the 4X4?

Am going to check brakes for draging just haven't yet.

 

Combination of both Get a good air filter, and good tires.

 

Some of the fuel around with bio in it will drop milage 1 to 2 mpg. I average in the 19's, my buddy averages around 21. Almost exact same trucks both driven hard. I use a bio mix and he doesn't.

 

New tires no difference, pretty sure no bio fuel mix around here.
Thanks

 

good filter,straight pipe exhuast,keep tires inflated and zmax will help ya out