Here's the skinny: Went to Alabama Christmas day, got about 17.5MPG. Came back this past Mon, and got from 14.3 to 16. Same road, same everything. Truck is a 2010 Silverado 4x4 with the 5.3, 6sp tranny, and 3.08 gears. Some of the gas was ethanol based, same as on the trip out. Last tank was real gas. Don't think there was much of a difference. Any ideas?