I drove up from Atlanta to Northern Kentucky and had the CC set on 65 the whole time. Did 552.5 miles and filled up with 9.476 gallons for 58.4 mpg! I was extremly pleased. My avg mph was 54 mph. I have a picture but dont know how to post it with my hone and am not near a computer.
Oops.... Didn't the dealer tell you not to go a constant speed or use cruise control for the first 600 miles I don't think that "rule" makes a lot of sense as the speeds of the ICE and motors will change with the ups and downs of the highway. I am impressed with your mileage!