P&G will always be the most effective driving method. However, if you're going into a head wind and are already at a decent speed the technique might be nearly moot. The purpose of P&G is to only operate the engine at its most efficient level - highly loaded. If you are already achieving a fairly high engine load because of the wind and speed you don't really need to glide as the engine will continually be loaded an adequate amount. I suggest watching the LOD gauge on the scangauge to determine when this is happening. By looking at a few
BSFC maps, it seems anything over about 60% load is putting you pretty close to ideal BSFC and therefore the benefit of P&G is drastically reduced.