I did an interesting mileage experiment with Mrs new car today as a result of feeling the fuel efficiency wasn’t as good as I would like using the adaptive cruise on mountain highways. There were two things it did differently than I would: it didn’t anticipate congestion soon enough to avoid braking (and wasting past gas); it reaccelerated too strongly when the road cleared - particularly notable on uphills. So...
One tank of gas by its rules. One tank of gas with me cancelling the cruise and restarting once I got the car back up too speed. Comparable net elevation for both. Exactly the same average speed for both. More total elevation on the tank I managed. I got more than 15% better mileage.