30 MPG is 'theoretically' possible if you use hypermiling techniques, drop weight, use thinner tires, ect... but real world for the LS3 camaro is roughly 17-19mpg.
My average over 110k miles is 19.6... ill have to dig and see if I can find my old mileage spreadsheet (stopped tracking every fill-up around 60k if I remember right..)
whoops sorry stopped at 40k... but on average this still holds true today.
Mileage details