Ok, have a 99 cummins auto, 4" lift 35" tires . cold air intake. 5" exhaust, 268000 miles, was getting 12 mpg running at 1400rpm always drive steady also has 3.55s and 14 psi lift pump . so got a pump from thourghbred diesel now only getting 10.8 mpg, added 2 stoke to fuel . Is it possible the timing is of truck does seem sluggish has good boost and does not smoke any help would be good . Buddy's truck gets 20 mpgs he has no cold air intake.

 

check for fuel leaks, also colder weather and winter blends can kill fuel economy

every diesel has worse MPG's in the winter time. use extra fuel during initial warmup due to cold temps, and winter blend diesel has more alcohol in it resulting in less potential energy from the fuel.

 

Your not running any kind of programmer are you? The lift and 35's sure arent helping mpg any.

 

There is no alcohol in diesel fuel and alcohol should not be added to the fuel, it is hard on the engine. Alcohol can carry water to the engine and the water is destructive in a diesel engine. Winter fuel does have less energy as it is refined to be more like kerosene.

 

methanol is in many diesel fuels...