The FCD portion will only go to 2,000 miles then starts over at 0, but the mpg will still keep tabulating. This is the one I use on my cross country trips. Don;t know about "A" or "B". I alternate between them on fill ups.
Both Trip A and Trip B do the same as the FCD, they reset after 1,999 miles back to zero, but the mpg does continue to keep track until reset manually. I confirmed this on a recent trip, when I knew the Trip A odo was about to roll over at 2,000 miles I watched and indeed the mpg continued the same at the rollover point. Later in the trip I confirmed the same fact for the Trip B odo. I tend to use my Trip A odo for mileage on a particular day, Trip B for my entire trip, and reset the FCD at every tank fillup when I'm taking a long trip.