You reminded me of something...
IT was flashing "service" so i followed the guide on how to reset it, which turned the trip to 0, but IIRC it then brought the numbers back to what they were, so now i'm not sure whether the miles (40 or so) that i calculated were true, or whether they were missing about 5-10 mile. If i had to bet on it i'd say they were true - which means the 28mpg remains.