07 Kenworth W900L hp and mileage?

Just bought an 07 W900L. Has a C15 Cat, 475 hp, 18 speed, 3.70 gears, and tall rubber. Finally got it on the road this week and so far I've been disappointed in the fuel mileage I'm getting. I haul grain and have a 50 ft Timothy triple axle and gross 88,000-90,000 lbs per load and have been getting about 4.2-4.3 mph and would really like to get up into the 5-6 range. I started looking into ECM upgrades, tuners, etc. I've looked into PDI Performance, Bully Dog, and Pittsburgh Power. Does anyone have experience with any of these or anything else? I like what PDI has to offer but it's quite pricey. Looking to bump power along with fuel mileage and I guess I'm looking for the best bang for the buck but want a quality product as well. Any help is greatly appreciated.