The best way to test it would be A-B-A over the same stretch of road on the same day. Pick out a stretch of road (say 5-10 miles). Drive it both ways at 62 mph, then both ways at 55 mph, then both ways at 62 mph again. If desired, you can keep on repeating. The difference between the two 62 mph runs gives you an idea of the inherant variability in the testing, as well as whether things are changing over time.
If there actually is a real difference, I would suspect the auto tranny. You don't go through a shift between 55 mph & 62 mpg do you?
There's actually quite a few threads already on this topic. Increading FE with increasing speed is very rare:
http://ecomodder.com/forum/showthrea...u-15182-7.html