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Hi,
This game is all about margins -- my exterior grill block (still temporary, now with transparent "weatherstripping" tape) gained me 2-3mpg. So not only does the constant fuel use on the highway go down by a greater amount, but also it is possible for me to coast down gentler grades, than I could otherwise.
The grill block (on my xA anyway) is about 2X more effective than my first take on rear wheel skirts. It is the most important part of the car -- since it is first to start to push the air. The pressure there is (probably) the highest, so smaller "flaws" make a greater difference than they would elsewhere.
The Focus' grill is recessed (maybe an 1"?) and the diamond grill itself is maybe ~1/2" deep? The lip that is formed at the top is ~1 1/2" which doesn't sound trivial to me. Some black Coroplast could be fashioned to form a flush surface, so the air flow has no lip at all would likely be much better.
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