Again, they're almost purely for aesthetics. There may be some fraction of an mpg gained from it due to some reduction in drag, but I'm willing to bet it's minimal in effect especially at city speeds. I'm sure many of the small design items don't add much in efficiency gains by themselves, but may be a "sum of the parts" advantage. If you're driving the semi trucks with enormous side-profiles and lots of wind turbulence, the side skirts and rear-fairings make a noticeable difference in fuel efficiency. They are traveling at highway speeds 90% of the time, so they reap a lot on the efficiency gains. You don't see those trailer additions on trucks used for local transport because there's not much advantage to them at 45mph.