A lot of different posts I have read here have said leveling it doesnt drop MPG too terribly much. However, those same post will say tires will kill your MPG. If you went from a stock 9" wide tire and went to a 12.5" tire, depending on the brand you chose, it could have added a lot of weight per tire. My Sierra SLT has stock 33's on it that are 40lbs each. There are some tires that are still stock size but weight 20lbs more each. That alone kill MPG without ever going up in size or width.
Plus, like rbdjr said, winter grade fuel sucks also.