I have a 98 1/2 24v Cummins and for some reason i get HORRIBLE fuel mileage. 11 to 12 and i drive it so easy. I travel about 23 miles one way to work and i use an 1/8th of a tank. Something has to be wrong. I havent purchased any gauges for my truck yet but i will give you a little info to the best that i know. This is my first diesel and i feel i got a good deal on it considering fuel is 4.29 a gallon here in wv. It only has 158,948 miles on it and i payed 5500. I thought that it would get entirely better fuel economy that what it does but so far it drinks the fuel but the power is great. It is all stock except for a straight pipe exhaust. The straight pipe couldnt hurt fuel mileage could it? I have friends that get 18-21 mpg in their powerstrokes and duramax's. I drive it easier than ever, 50 to 55 on my way to work. Someone help me out

 

did you check the air filter?

 

Check tire pressure also, set them at the max pressure imprinted on the sidewall.
Sounds like a pretty good deal, even better if you can improve the milage.

 

check the fuel filter. does it have 4.10 gears with a 5 speed or automatic? ive been driving a 5 spd with 410s all week and its been sucking it compared to my automatic daily driver

 

its a 5 speed with 3.73 gears. The fuel filter and air filter are both brand new. I dont know what could be causing it.

 

Fuel pressure could have something to do with it. If your really starving for fuel your going to have worse fuel mileage. My 98 was same way as your when I got it and the raptor I put on it actually helped fuel mileage surprisingly

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Weak vp/bad injectors can be a contributing factor also

 

98.5 5 speed with 4:10 get around 16mpg out of my rig, remeber its a truck you don't have to try and kill in every gear be smooth running any additive in your fuel?

 

Valve adjustment. A Cummins needs a valve adjustment around 75K miles. If they have never been adjusted your gas mileage will be horrible at 150K miles.

 

Could be the VP44 dying. Poor fuel mileage is one of the signs.

 

I would first check your fuel pressure. Scan it for codes.

Luke