You got some issues if that's the mileage your getting with a 305. My Tahoe with the 350 and fat tires all around did pretty good. Never really calculated it but guessing mid teens. My 08 Tundra lifted with 35s, 5.7 and 4.30 gears gets 11.5-12, mix of highway and town. I live on top of a hill/mountain (many call it a mountain, but I've been to the Rockies, these are hills lol) but the terrain here is all hills. Air pressure plays a big part. Check em, maybe pump em up more. Fresh tune up, and as others stated check for anything dragging while rolling. Plugged cats or exhaust sure won't help.