I did this...to prove a point to my wife....we made two full trips around the lake (one way then back with essentially no traffic on road) I let cruise handle the first trip, at the speed limit and I handled the second... A bit more aggressively... (Not like a pissed off teenager, just an average relaxed drive for me) what do you know...I got the prize . Of course sometimes its better to be happy than right lolMy results were similar to yours, Ray. I live in a hilly rural area. When I bought the truck in 2010 I quickly realized that the cruise control wasn't going to win any mileage rewards. I can get 2 - 4 mpg above the CC driving the 2 lane country roads without much effort. I can get a couple of miles above that when I work at it. And when driving aggressively in a hurry I drop to 16 - 18. I think the window sticker mileage figures are very achievable if you keep your foot out of it.
And if I drove like a cop it would probably drop to around 10 - 12 mpg.