That depends. I assume you have the 350 and probably have the 3.73 rear end. With any mpg values that one might get, it can vary depending on a lot of factors. Engine size, rear end ratio, driving habits, ac on/off, "tuned up" engine, air filter dirty/clean, etc.
What kind of mpg values are you getting? Without knowing any details other than what you provided....I would expect maybe 15-17 on highway and 13-15 around town.
Hope this helps.