What year is your balt? 05-06 don't have the instant mpg in the DIC only the average (I know for sure my 05 doesn't). If you really want to know though, I think they make ones that plug in where the code reader does that show instant mpg.
My 08 has instant but I use an ultraguage to watch my fuel consumption more closely. If you're interested in your instant mpg and/or saving gas I would suggest one. They are a little annoying to get them accurate but overall I like mine.