My 2020 GT Premium has a screen in the Mileage Application that displays the Cylinder Deactivation Status. Even at 70 mph when you go down a hill it turns the light off #1 and #4. If the hill is steep it will eventually turn off the light on all 4 cylinders I guess with a fuel cut off. At the bottom of the hill all four lights come back on at the same time. The story of CD working only under 40 mph does not seem to apply to this model. It is also getting about 2 mpg better with a new tight engine vs my old 2016 GT that has 60,000 miles on it. I am unable to detect when CD is active except by looking at the screen. So far so good...