Hi & welcome to the forum,
Good highway economy = good aerodynamics. You need to do an upper grill block, to start with, and smooth wheel covers will help a lot, as well. Here's the thread I started on my xA:
http://ecomodder.com/forum/showthrea...mods-2969.html
Those are the two biggest improvements for me. A couple of the other helpful mods were the fog light covers, and the zigzag tape at the top of the windshield. My high is 47.5mpg and during the summer I averaged 45.5mpg. My smooth wheel covers have a good chance of getting me above 50mpg (I just added them when I switched over to my snow tires). My goal is to stay above 40mpg this winter -- and so far, so good!
The only Cd that I've seen for the xA is 0.31, btw. Yes, the Prius does well with a lot less effort -- I drove one for two tankfuls, and got ~52.5 and ~53.5.
Oh, and you probably should get a ScanGauge II -- that has really helped me a lot! And having a coolant temperature gauge also helps a lot; my grill blocks (~3/4 of the entire grill is covered) makes virtually no difference in the engine temp, at maximum, though it helps shorten the warm up time.