Did you achieve this value (35-37mpg) with the cruise control on? If you had the cruise control on, did the RPM's exceed 3000 on the uphills?
My experience with highway driving with and without the cruise control indicates I do not have to worry too much about a difference in fuel efficiency. However, I have not gone "over" the six mile long uphill on the Pa Turnpike between the New Stanton and Somerset exits. I may this weekend (Saturday) as I would like to attend a historical CCC presentation at Laurel Hill Park which is just east and below Seven Springs Ski Area.
I had the cruise control on most of the time, RPM's stayed pretty low. I took the scenic route trip to cleveland instead of the highways, on the way home i took the highways and didnt get as good of mileage because i had to keep up the speeds of 60-70 mph. Although it is based on one trip i found that speeds between 45-55 mph gave me the best mpg.