You can try out fueleconomy.gov? I think they say something like 15 MPG city, 19 hwy.. combined 22, something around that. I have read from 13(or less) to 17 mpg hwy. I think it all depends on how you drive the S5.
There isn't too much difference between the mileage of the S5 and A5.. which is pretty cool
There isn't too much difference between the mileage of the S5 and A5.. which is pretty cool