This isn't scientific (not enough miles), but I made a repeat trip to my friend's house in St. Peters, approximately 30 miles from home, mostly highway. On the last trip, I was able to get 45.5 mpg. I figured this might be slightly elevated by pump error, as this is incredible economy. This trip differed in two ways: it was hotter outside today, and this time I had my wheel skirts and grill block installed. 47.2 mpg is my new high; although the results aren't conclusive, they do suggest a gain of ~2 mpg at 55-60 mph. If this is correct, they will be most useful when traveling on faster highways to and from Kansas City (70 mph).
A side note regarding recent sensationalism in the news and AAA's press release denouncing hypermiling...
Some people might point out here with my 70 mph comment that they've heard that hypermiling involves driving slower on highways. I don't EOC or coast, etc. on highways except on offramps. My highway hypermiling is to drive at the speed limit (which does end up being slower than traffic), in the right lane with the windows closed. If I can find a large vehicle to follow, I do so from at least 5 car lengths.
Hypermiling has gotten a lot of bad press recently, and I make sure that my driving doesn't overly annoy other drivers or pose a safety concern. My EOC driving is on city streets when approaching stoplights/stop signs or when on a long downhill. I don't camp in the left lane and "police" other cars to drive the speed limit, and I don't EOC to speeds more than 5 mph below the speed limit if there is a car behind me on a two lane road. Doing so is counterproductive to the fuel economy movement, and I'm hoping to get more people on board.
Lifetime Fuel Economy: 39.49 mpg
View my fuel log here: http://ecomodder.com/forum/em-fuel-log.php?vehicleid=717