Stock intake is good for about 500hp and helps you maintain the warranty for the motor. An exhaust might gain you 1-2 mpg at best. The best way to up your mileage is to drop back to stock height and tire size. Since I'm guessing that probably won't happen you'll get the best results by putting an egg under the skinny pedal and trying not to crack it. 12-14 is about the best I would think you'd see with the lift and wheels. I'm running about 15.5 mpg in town and 18 mpg on the highway and my truck is stock.