First, is your Suburban 2WD or 4WD? Half-ton or 3/4-ton? Same year range as this one? Second, where are you? In CA, where the traffic light is the unofficial tree and the state pastime is gridlock, all that stop-and-go driving kills even half-decent mean mileage.
It's a 99', 4WD, 3/4 ton with a 5.7 liter V8. It was purchased and driven in California for several years and hasn't improved it's mileage since it was moved to a red state.