"It gets around 8-10mpg in the city"
That's baloney, I never get that bad a mileage even when pulling a two ton horse trailer.
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.
I love the internet. Any piece of data you'd ever imagine is available, if you use the right search terms: What's your Sub's Gas Milage?.
The general concensus seems to be: 8-10 in the city, 13-14 overall, 17-18 on the highway, 13-14 when towing.
Happy now?