high mpg with wheel covers removed

I have removed the 15" wheel covers from my 2012 Prius since they interfered with accessing both the lug nuts and the tire stem (what a stupid, cheap wheel cover design), and replaced them with 2.5" center caps to protect the axle threads from corrosion. Although I didn't rigorously compare mpg before and after removing the wheel covers, on a recent out-and-back trip with the new center caps installed, the Prius averaged 56.5 mpg on a hilly, 240 mile course, at a speed of 66 mph (dry road, not very windy, air temp about 75F, cruise control employed, no hypermiling techniques). I was quite pleasantly surprised at the high mpg (even better than I typically get with my 07 Prius on that same trip), and can't imagine that the mpg could be much better with the wheel covers on.