My smart never shits on the Highway. On the on ramp sure, but by the time I'm on the Highway I'm in 5th and it wont shift again till I exit an Hour later. Doesn't get smoother than that
Just ignore Franoc; he's apparently run out of his meds again.
Mileage improves as the car gets broken in. Somewhere around 3K miles you should see a big improvement. Of course, driving in "D" and keeping speeds below 65 mph helps too.
There are many factors involved with mpg. Keeping the vehicle in tip-top shape helps but you most also consider that a vehicle that is weighted down will not do as well as one that is kept relatively empty. Leave items that do not need to be in vehicle at home. Add on accessories will also add weight (i.e. subwoofer boxes, amps, heavy floormats, and such.)