Questions about 2000 GMC SIERRA?

I have a 2000 GMC SIERRA with a 6' lift with 35' tires. I'm only getting about 7-8ish MPG. I'm wondering if anyone else out there is getting about the same? And if this sounds pretty normal. It's a 2500. It's also starting to get some transmission problems, it's at a little over 150,000 miles. Anyone know what it would run me to get a new trans put in it? Or if it can be done my me and some buddies? All your info is good, thank you.