Some autoboxes have it, some don't. Those that don't, you need to manually shift to 3/S/L gear to engage it.

Waja engages it automatically on downhills, Persona 4AT doesn't, Vios doesn't. All Proton CVT does it.

For most part it is a safety feature, like when you go down from genting the road signs tell you to engage low gear - it's the same thing.

Rolling down hill with high RPM doesn't consume more fuel than idling. High RPM caused by engine braking is not the same as high RPM caused by throttle.

You Waja fuel consumption is caused by simply because it is Campro.

I used to own a Mitsu Waja and it does the same thing, downshifting when going downhill.

This post has been edited by dares: Oct 23 2021, 10:32 AM

Not just theory. If you have OBD scanner, you get get fuel flow rate from the ECU. When engine braking downhill with no throttle, the fuel flow rate is 0.

Yes BMW coasting mode will disconnect the transmission from the engine.

But by doing so, it will need to continue to inject fuel to keep the engine running compared to engine braking which requires 0 fuel.

On the flip side, engine braking will slow down the car and you will need more fuel to re-accelerate back to speed, which the BMW coasting mode doesn't thus saving fuel.

It's a give and take, and either one would work depending on the condition.

But knowing Europe's strict emissions regulation, this coasting function is probably designed to meet those requirements instead of being a genuine effort to save fuel. Like those silly cylinder deactivation tech.

This post has been edited by dares: Oct 25 2021, 11:10 AM

I think for flat roads or slight downhill gradient, the coasting function would be more efficient.

But for steeper downhill, engine braking would be more efficient because we are using gravity (which is free) to move the car forward while using no fuel.

Maybe it is not done because even with WOT the TB and valves are still restrictive and the pistons will still be pulling some amount of vaccum? Definitely not as much as closed throttle, but I'm guessing enough to cause deceleration.