At that low of an RPM, it is easier to keep the engine spinning by using all six. The amount of fuel is minimal, but consistent. I would guess that firing just 3 or 4 cylinders increases fuel consumption because of the extra "push" each cylinder would need to generate to compensate for the ones that are at rest.hmm, wondering any reason why not shut off couple to boost city MPG.
Pretty cool! Anything to squeak out a few more mpg's.Also my guess is that it is much of a hassle to enable VCM on idle (vibration, launching effort and its emissions, etc.) for such fuel small savings.
But the overall idea is being taken by other vendors (albeit smaller engines), let's see how it develops, it reminds me of gas golf carts
MAZDA:Idling stop technology | Environmental Technology