Can some one tell me if this is normal.... 2004 Superduty F-250 diesel 65000 miles. Just did a 3000 mile round trip. All I can get is 15.5 mpg on paper "Manually".. is this all I should be getting??? 3:73 Gears, 2.5" Lift, Edge tuner/ attitude set on #1, Cold air intake, 5"banks turbo back...