If you sit at a red light or dont move the ENTIRE 5 minute segment, you haven't gone anywhere...0 miles, 0 gal; so the the gauge will read 0. But, if you are in EV mode and inch forward and stop for the rest of the 5 minutes, you have gone .00001 miles and used 0 gal. you will have "99.9 mpg" because that's the maximum the gauge reads. I don't think it has anything to do with being in Park or Drive.