Smoothness is absolutely key.
You probably gain nothing from coasting downhill to a higher speed. That higher speed causes significantly more drag, and you're losing energy to that more than you would if you kept a steady speed. Or that's my theory anyways.
Edit: this assumes you're using regen. Coasting is absolutely better for an ICE vehicle. Higher speed without input throttle is always better in their case.
Speed steadiness is likely the difference for people who get better efficiency with cruise control. It doesn't really overshoot the target speed, however a human often does by a little bit. That little bit is disproportionately worse in terms of aerodynamic drag, so you actually lose a fair bit of energy over time.
That said, cruise control isn't as smooth as you can get. Drivers can definitely do better than AP.
I agree. All cruise controls will "step on it" to maintain speed going up hill. This always seemed wasteful to me. Can't I have an intelligent AutoPilot that keeps the same energy consumption - within reason - no matter how hilly? Downhills are an exception - don't go over my set speed. I assume trying to maintain 120 KPH / 70 MPH uphill wastes many KM / miles of equivalent flat driving. And AP is never so smart to accelerate a bit before the hill and back off while climbing.
Thoughts?
My thought is keeping the same power to the motor as if you were going 120km/h but up a steep hill would maybe result in a steady state speed of 30km/h. You have a very heavy car that you're hauling up a hill, it requires insane amounts of power to do so at a reasonable speed.
Try keeping the black bar constant some time prior and during an uphill. Notice speed is likely lost quite rapidly.
My experience with cruise in a Model 3 is that it is rock solid, very rarely fluctuating by +-1km/h. Most gas vehicles are significantly worse, especially with the lag and momentum of the drivetrain.
I rest my foot gently on the pedal of the brake, in the event I need to react, more often than not, I witness very little regen, in the status bar, and feel the brake depress. Stop and go traffic is one of the instances I see this the most. When I am operating, I take the most advantage of regen, in stop and go, and very rarely have to apply the brakes.
In stop and go and likely especially with the Roll stop mode, yeah TACC/AP tends to use friction brakes a lot, especially since its overall not very smooth in such traffic.