In my Cobalt (and then G5) the advertised MPG was 24-30 or something?
I averaged 26ish pretty consistently.
In my Z I seem to be steady at 21MPG.
I do 80% city-esque driving with the rest being highway. If I was incredibly gently and drove it like a Prius I'd imagine I could get to 24-25.
That will never happen though.
edit: also it's pretty sweet having a 380+ cruise distance.