I was at the dealer pricing out a GCSRT8 the other day and they printed off a window sticker that says Hwy fuel economy is 11.4 L/100k. When I check fueleconomy.org, the number they provide is 13.1 L/100k.
Regular cruising on the highway, I'm getting around 14.3L/100km which is about 19.75 mpg (imperial gallon) or 16.44 mpg (US gallon). So if it's 11.4L/100km, that's not so bad I'd say. That's about 24.8 mpg (imperial gallon) or about 20.75 mpg (US gallon).