[Overdrive]

You may want to have the car checked out by someone other than the dealer) to see if the ecu has been tuned for the exhaust it has. It's possible that it hasn't, and that could affect mileage for sure. You do have to accept that the car's not going to be terribly efficient, and the ill effects of driving the car like an economy car at low rpms ae not worth squeezing out a few more mpgs. If the car needs a tune it could help you gain a few mpgs and smooth out the fuel map. The car's fun, for sure, but like others, I don't go beyond 3rd gear in the city, and I don't use 5th (or 6th in your case) until I'm above 70mph. My old base gets a pretty consistent 20.5 mpg, by my own calculation, not the car's, with mostly city and turnpike driving, but it's a much smaller engine than your 08 S, even if it's from an era of much lower efficiency compared to today's cars, that still likely plays a part.

If you can, try to make your next long trip mostly highway, and try to bring it down 10mph on the highway to 70mph. 10mph makes a huge difference in fuel economy no matter what you're driving, trust me.

You may also want to try filling up with the high octane from a few different gas stations and see if there may be a brand that your car likes more than another.