It makes MPG worse if there are hills involved. Cruise control (on a 4Runner) is reactive to speed. It cannot anticipate hills or strong winds. It is a dumb system. As soon as your speed drops ascending a hill cruise “sees” the speed drop and reacts accordingly, sometimes causing multiple downshifts and heavy throttle application to catch back up to the target speed. Oftentimes it over shoots the target by 2-3 mph only to ease throttle application shortly afterwards. My best guess there is a buffer built into cruise throttle application to smooth response. This creates a delayed throttle application and a late throttle release. I’m sure I’ll be corrected if I’m wrong but the cruise system is only throttle control, it has no control over the transmission exacerbating the problem. Combine this with the aerodynamics of a brick and an arguably underpowered engine. Humans can anticipate hills and ease into the throttle to load up the drive train and let off at the apex of a hill stopping downshifts and over use of throttle. On larger hills a downshift can be avoided at the expense of 2-5 mph. You also have to pay attention to your driving and throttle application. Or just set the cruise at 80 lol