Well I have a 93 chevy K1500 (half ton) extended cab 4x4 auto v8 350 TBI and I average 13 mpg on a good day in 2wd. And Im also running 33 inch tires. I also have a fresh tune up and don't do alot of highway driving. So I would say any where from 11 to 15 mpg.
1991 Sierra no carburetor used unless your have a older engine in there, as for the mileage, depends on your final gear drive in the rear end and if you have manual or automatic transmission.
Wish i can give you more information but, don't have much to go on.