Gas mileage with today's V8's is a moot point. All LSX motors (that's LS series motors like LS1, LS2, LS6, etc for you noobs) are getting anywhere in the range of low 20's for city and high 20's to low 30's for highways. With the 6 speeds that is which is what I own.

Hell Cobra4b got like 500 miles to a full tank of a gas on a trip he went on and had 400RWHP on tap all the way there in his Z06.

And just to think. That's using engineering from the time before your parents were born.