[SlickShift]

I rarely drive in Eco Pro mode, but I do notice that the car "coasts" when off the throttle - essentially putting it in neutral. To my understanding though, neutral requires spark and fuel to keep the engine alive (albeit very little).

But if the car is in gear and off throttle, no fuel is needed since the engine is connected to the wheels, thus spins at a relative RPM. Wouldn't this mean the car would be using MORE fuel by "coasting"? Can someone please explain where I'm wrong since I'm sure the BMW engineers are much smarter than me?

[Gabrola]

Keeping the car in gear will cause some slight engine braking. So while the car will not be using any fuel while you are off the throttle, you will need to get back onto the throttle much sooner. With the car in neutral the car maintains momentum very well and will actually gain speed on slightly downhill roads.

Eventually the fuel used by the engine idling while coasting ends up being less than what's needed to maintain the same average speed in gear.

[Maynard]

And with the fuel injection, it does not really NEED to keep using gas like with a carb, no matter what the vacuum or rpm. They may just fully deactivate until you get back on the throttle, rather than idling?

[Questofthetune]

Originally Posted by Gabrola

Keeping the car in gear will cause some slight engine braking. So while the car will not be using any fuel while you are off the throttle, you will need to get back onto the throttle much sooner. With the car in neutral the car maintains momentum very well and will actually gain speed on slightly downhill roads.

Eventually the fuel used by the engine idling while coasting ends up being less than what's needed to maintain the same average speed in gear.

The coasting gets me all the time since I'm so used to the engine brake effect- feels like I'm flying straight into the back of a car sometimes haha

[TajoMan]

It is fairly new technology called "Neutral coasting".
On flat road, neutral coasting saves more fuel than in gear. (due to engine brake reduces the coasting distance)
On downhill though, car uses gear for engine brake and fuel cut.

[DRitch]

Originally Posted by TajoMan

It is fairly new technology called "Neutral coasting".
On flat road, neutral coasting saves more fuel than in gear. (due to engine brake reduces the coasting distance)
On downhill though, car uses gear for engine brake and fuel cut.

It's always seemed strange to me that this is a new technology, especially for an automatic with a torque converter, since automatics (at least in American cars) did this in the '70s.

[TajoMan]

Originally Posted by DRitch

It's always seemed strange to me that this is a new technology, especially for an automatic with a torque converter, since automatics (at least in American cars) did this in the '70s.

Maybe they didn't do it for fuel economy before.

It is proven recently (means last 5~10 years) that neutral coasting saves more fuel, mainly due to difficulty to measure the saving accurate enough.

Measuring fuel economy accurately and consistently in real world is very difficult.

[BentZero]

I'm assuming that this is in an automatic car? I've not noticed this the 1 or 2 times that I was in eco pro in my 6MT.