2005 cummins isx poor fuel mileage no power

[reholt1975]

I have a 2005 Int. 9900I with a cummins ISX 435 hp. I am getting 4.2 miles per gallon and it will not pull a hill no matter if you have 6500 lbs on or 45000lbs on. I pull a 53 ft dry van mostly highway and keep it at 65 mph. I run the engine at night for comfort. I know this truck should get better than that as my 99 freightliner classic with a c-12 was getting 6.5 to 7 mpg. The shop at the company i am buying this truck from said to put a wing on the truck but i know for a fact that will only give a half mpg increase at the most. Please help if you can as I am about ready to give them this truck back.

[Fernando]

Give Pete a call at 724-360-4080.