Actually I have no clue how large the tank of my CTS and Neon, resp. is. But thanks to the manual trans I achieve an average of 10,2 l/100 km (23 mpg) for the CTS (with a lot of city driving) and 8,1 (29 mpg) for the Neon.
The picture below was made after driving on the Autobahn with cruise control and 140 km/h (87 mph).

(25 mpg)