bad gas mileage

Ok, I just got back from Afghanistan last month and since I started driving my truck again, I've noticed that the gas mileage has been poor. I've cleaned the air filter, cabin air filter, replaced brake pads and rotors, replaced the tires (265/70/17) which was the same size and style as previous. Before I deployed I was able to get about 330 miles to a tank of gas on all highway, now i'm getting about 270 max and the driving has been the same. any suggestions as to what is causing this? I have an 06 with 78,000 miles, 6 speed no lift.

 

Part of it could be the increased ethanol in gas this time of year. I've been seeing a noticeable decrease in my mileage over the last month or so with virtually the same type of driving and my truck only has 15K miles. Also, have your plugs been replaced since 30k miles?

 

Yes I had them replaced at 60k I'm looking at the 80 k service here in a week or so.