Well, pup, driving in New York is not driving, is it ?
I think they call it parking for the most part...
and at zero mph it's hard to average anything much over 0 mpg

I can tell you that the mpg for both the sport at 20 mpg (the wife chides me all the time how my driving drops her average below 20 mpg!) and the 350 at 16 mpg around the dfw area are what the displays show, but we are not in stop and go traffic 90% of the time... nor do we double check the displays since we really don't care about mpg or else we would have neutered ourselves and just got a prius !