no facts, just experiences: Same thing happens with our minivan, lots of up and down shifting, especially at overpasses and bridges, where it might downshift 2 or 3 gears...instinctively I believe that has to hurt mileage, and I believe that it doesn't do any good to the transmission either....my solution is to take it off cruise control when I see an overpass or hill coming up, putting the cruise back on on the downhill. I am pretty sure that my mileage improved after starting to do that. In my little car (Mazda Protege5) I use cruise all the time because it has a manual transmission, I just keep an eye on the RPM's on hills and downshift if the RPM really drop off.