Your truck is geared a little low for a DD15. We would normally want to see that engine run at a lower RPM. How many miles are on this truck? I'm assuming the emission systems have never been cleaned and inspected. If this were my truck I'd get it in to Pittsburgh power and do an emissions cleaning/ tuning. Also have them check the charge air cooler for leaks and talk to them about fuel mileage upgrades like the fleet air filter, Tires with low rolling resistance are going to have a big impact in your operation , so when you change tires get the lowest rolling resistance possible. If you have the time try running 55 mph and track the improvement on fuel economy. Slowing down with that gear ratio and the DD15 should give you a pretty good improvement. Hope these ideas help you out. Keep us posted on you progress.