JPL: Don't know why your highway mileage is so low.. is it highway or freeway? Do you have a lot of stop lights?
On freeway trips I've averaged about 27-28 mpg with steady driveing at about 70 mph and using the cruise control.
My short trips/commute in town are not too great, about 15-16, but I tend to have a heavy foot and the acceleration factor hasn't worn off.
It would be interesting to fold the side mirrors in and see if it's quieter and gets better mileage, but I wouldn't risk the danger of no side views or getting a very expensive moving violation ticket.
I can just see the scenereo with a highway patrol pull over:
O Can I see your drivers license sir?
R4 What's wrong officer, was I doing something wrong? I'm sure I wasn't speeding?
O No sir.. but I noticed both your side mirrors are folded in?
R4 Oh... that.. I was seing if I could reduce my gas consumption a bit.. funny, I didn't even see you coming..
O Uhh... ya gotta be kidding, right?