Mine sits right on 31 mph. I would think it would change a mph or two after some long highway cruising, especially with only 4,000 miles on my Stang (I have never reset the average mph since I bought mine). I guess if I'm not interested in what my average mph is from the day I got the car, I could reset it and see if it really works.
On a side note, my MPG reading is more like 0.7 mpg low than a full mpg, although my last tank was almost dead on. I suppose the accuracy may depend on what tye of driving you had on the current tank.