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I used to think a manual transmission would always be more economical, but I am rapidly changing my mind.
3 Months ago we bought a 2017 BMW 420 diesel automatic and the gearbox is most impressive. When coasting, the engine is somehow disconnected from the drivetrain and drops back to idle speed regardless of road speed. There is no engine braking and the car will coast more easily and much further than any manual I have driven. I am still getting used to how early I can lift off and still easily coast up to junctions. In a manual car I often coast with the engine off, but the auto at idle speed seems to coast more easily than that. How it does so, I do not know. It is this improved coasting which I believe gives improved fuel economy.
I should perhaps point out that the car has 3 "modes", sport, normal and eco. The gearbox works as described above in eco mode, but I cannot say if it does so in the other modes as I always drive it in eco mode. The best economy I have achieved so far is 3.8 litres per 100 km.
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