I took a long trip over several weeks this summer, and one thing I noticed about cruise control is that it had a tendency to suppress the MPG on relatively steep downhills. Several times, it downshifted fairly hard and drove the RPMs way up as a way to maintain the speed. I eventually got in the habit on long declines to take CC off and just let the truck run with mild braking when needed. The average interval MPGs consistently increased as the truck was in more of a coast than cruise mode.