I agree with the above post. My 300SD does 24mpg with mostly city driving and a/c on, 26mpg on highway in cool weather. And there is a definite drop off in fuel efficiency when driving over 80mph for long periods of time, so 55mpg would probably get 27, maybe 28mpg. And I mostly fill up at the same station, always fill until I can see the diesel so it is about the same place, and the numbers are really consistent. So, no way does any stock 300SD get over 30mpg.