The STS V8 is officially a "gas guzzler" and a tax was collected at the dealer sale. My original window sticker which I still have says it was $1000. I see about 23-24MPG on the highway during the summer and 10MPG in winter in town.
I live in a small town, but never see an average below 15 or 16 for in town driving. My overall average for mixed driving is 19 and a fraction almost always. My hiway driving is 26 and the sweet spot is aggressive driving and cruise at 65 to 70 mph. These cars are not set up to be babied.