Here's my two cents:
I drive a 2008 Impreza Outback Sport (19/27?) and have put on 94,000 miles. 90% of my miles are highway driving and have cruise control set at 70mph. I've been averaging about 26 mpg since I bought the car.
My 08 Impreza is in the shop so I've been renting a 2013 Impreza for the past 4 days. Again, I do 90% highway with the cruise set at 70 mph. I've been averaging 33 mpg. Its not quite close enough to the advertised 36 mph as I would've liked but I am confident that it is because the rental only has 1300 miles on it.
Food for thought.