I reset the MPG display when I fill the tank. So far, I've kept 'Trip A' and 'Trip B' the same (also reset at each tank), so I don't know if any of the other display calculations are affected by it. I suspect that they aren't, however, since most of them can be reset separately.

For the mileage on individual trips, I calculate that from the change in the display mileage (and the distance driven).