I did a test for a few tankfulls this summer. I call it my "Drive like an ass" test. Prior to this test my 300cd non-turbo has consistently gotten no less than 21mpg in the city driven with economy in mind, steady acceleration, coasting to stoplights, etc. If it gets 21mpg normally while babying it, what will the mileage be if every stoplight is a drag race? Driven like an ass, mileage increased to a rounded-down 22 mpg in the city, average of three tanks. City speeds are less than 40mph with many stoplights. Maybe it was a fluke or me not using A/C (I didn't anyway, it was broken), or driving stupid got me better mileage than smart.
Highest mileage I've gotten out of my 300cd is 24mpg, 55-65 mph local interstates.