New flatter aero wheel covers:
16" aluminum pizza "trays" (~$3.29 each from Amazon.com) pop riveted to the plastic wheel covers I got at K-Mart (~$17 for 4). There is a slight gap at the very edge, so there is probably a little air flow, and hopefully they do not trap dirt and brake dust.
Here's the xA as it currently is.
The most recent tank average was 54.8MPG; a new record for me! I'll see if these new covers help over time, and hopefully they stay on the car...