I put some hub caps on my car (not moon caps but close) and the gain/loss was pretty much immeasurable. they do look pretty good so I left them on.
the people that see gains are already way up there in the mileage anyway. for us regular folk, there are larger gains elsewhere.
*edit*
if I were you (and i did this myself) I would buy a scangauge. it was my first mod and made a world of difference. it is a device that plugs into your 0BD-2 port and tells you in real time what your mpg are and gives you feedback of what the car is doing. I have used it for many different experiments as an information tool. this will help you change your driving habbits.
www.scangauge.com it is a touch on the expensive side $170ish but mine has paid for itself several times over.