You'd need to drive the automaic in semi automatic mode for it to be as efficient, right?
The method of action is different. An automatic transmission using a torque converter loses more energy in the form of heat than a traditional manual gearbox.
The only gearboxes that have the same efficiency regardless of whether or not they're manually shifted are true automated manual gearboxes like BMW DCT or other such auto-magic boxes that use clutches rather than viscous (liquid) coupling to facilitate gear changes.
https://en.wikipedia.org/wiki/Torque_converter#Lock-up_torqu...
There should be more coasting with no gear engaged and low gear low rpm for low loads, than what automatics chose in practice.
But I don't have any data at hand for practical fuel efficiency and modern automatics versus manual.
I think it is accurate enough to think of each revolution as a fixed cost for 'low' or idle power.
At idle power the motor should probably only be engaged when also braking.
a cars' stability is aided by engine braking; if that isn't there it must be accommodated by knowledge of such a fact by the ABS/TCS/VCS systems, or a skilled driver that is expecting it.
A regular car that expects engine braking, thrown into neutral and coasting down the highway is a more fragile handling and car-dynamics situation without these precautions. The Mercedes Glide Mode accommodates this by using the ABS/TCS systems to keep the weight where it needs to be regardless of applied engine torque one way or another; the driver does not experience a differently handling car.
Or is there some effect in the rubber from engine braking that make the front wheel more "prepared"?
Mercedes Glide Mode for example does this automatically to increase MPG (allegedly).
The latest ones do, being integrated with GPS and a terrain database to anticipate changes.
for at least 20+ years now BMW/Porsche/VW also use the onboard IMUs and gyros and wheelspeed sensors to determine whether or not a shift should happen , say, in the middle of a long high speed banking turn.