What kind of gas mileage should my 2000 GMC get?

I have a 2000 GMC Sierra 2500 with a 6' lift, and 35' tires. What kind of gas miliage should I get? Right now I'm getting 7-8 MPG. Also on top of that the tranny shifts hard. It's got over 150,000 miles on it. It only will shift hard when it starts getting warmed up. Should I replace it? Or cut my loss with the truck?