Got my first 55+mpg today....

I've got 2600 miles on Greenie now and the mileage seems to be inching up. As mentioned before I don't stare at consumption screens and don't P&G, etc. Every week my wife and I take a 60 mile round trip to buy organic foods/groceries and always drive the same route. The route is essentially secondary country roads that have no flats but consists of rolling (but not very hilly) terrain. I just put the cruise on 62 and stop only at three or four 4-way crossings. Our mileage has gone from 49.9 to 55.1 today with no changes in driving habits. What's the deal?? BTW, when I pull in our driveway and look at the techie screen I almost always have green bars at the end of the trip.