IMO if cruise saves gas depends on many things. It depends which car, how many gears and on weight.
Lets say the car when its on cruise at 75 mph it'll be at 2 rpm but when going up a hill depending how heavy it is, it'll need more power so it lowers the gear and the rpm jumps up, that uses more gas.
Another thing is a few cars have only 5 gears like some manual cars, mazda 5, chevy aveo etc so when going over a certain speed and its on us highest gear it will just constantly go up in rpm.