Take off the factory muffler and run a straight pipe from turbo on back,then put on a good Air Box and Filter like AFE,ect. Get more air in and push more out=power and better mileage. These are the two biggest HP and Mileage gains I performed on my 7.3 PSD for the least cost. If you have unlimited funds MBRP has a new 4 inch 7.3 exhaust system including the down tube off the turbo and stainless steel straight pipe all the way back--a good investment. PS: What is your rear gear RATIO?? Mine is a 4X2 with a 3.73 and I get 20-21.5 mpg on the road at 65 mph. I get 13-13.8 towing my 13K lbs,40 ft Fifthwheel with that gear. A friend of mine has a dually f-350 7.3 with a 4.11 gear and gets about 4-6 mpg less. Good luck==there are many other updates that will help to a lesser degree,but they all add up !