I have a 07 Silverado 5.3L crewcab 4x4 and my question is how can I improve the gas mileage. I get at best 15 miles to the gallon driving mainly highway most of the time I get just under 14 unless I haul a trailer with a motorcycle on it I get 11 miles a gallon

What you are getting is within a mile per gallon of what this vehicle gets....nature of the beast.......about the only way to increase mpg, is your own personal driving habits....

Sounds a tad low to me, how many miles since a tune-up and what is your rear axle ratio? Stock tires or taller ones? The reason I say a tad low is that with a 2005 K1500 extended cab with the 5.3 and 3.42 gears I achieved right at 17 mpg on a 3000 mile cross country trip from Wa. state to SC almost a year ago and that was with 75-80 mph interstate speeds most of the way. I can get around 19 mpg highway here in the relatively flat land with more moderate 65-70 mph.