Cruise control on my car actually gains me about 1 mpg. This is specifically because the car "knows" that the speed won't be changing much so it locks the torque converter completely and drops the engine RPM by 100~200. Many of those things are direct results of the mediocre Ford implementation of the 2005 Five Hundred CVT and the fact that it predates Adaptive Cruise Control, but it has a physical reason and follows directly from new assumptions in the code only viable from the state I put it in.