Somewhere around 2000 rpm about 40mph seems to work for the xB if you are going for distance - slower is ok for short periods of time 5th at 20-25mph if really level roads.
I find that 40mph in 5th gear is optimal for the Accord. I can do a little better at 30-35mph if the road is flat, but then the engine can't make enough torque to get over a hill and I end up downshifting. If I could drive 40mph over a reasonably flat road without many stops, 50mpg+ would be no problem at all.