I would think 15 is about as good as your going to get pushing that pile of bricks through the air with the big aggressive tires, regardless of what is under the hood. It takes a certain level of power to move that mass through the air, and power consumes fuel. Smaller all season radial tires, a more road friendly gear ratio, and devices to smooth the air flow might gain you a few more mpg, but I think your about at your limit without radical changes. There is a cost to having fun, in your case, its fuel mileage (and probably a few broken parts along the way). Gene