Im going to try that nitrogen in the tires to see if that helps at all...
My guess is you'll see pretty minimal gains as the benefits of nitrogen are going to be pretty tangental to gas mileage. There are a few purported benefits to nitrogen filled, but they're pretty slim.
There's some argument about the moisture being a factor of benefit. Powerslave pretty much hits the nail on the head for everything else. Here's a pretty good article on filling tires with nitrogen.
http://www.straightdope.com/columns/read/2694/is-it-better-to-fill-your-tires-with-nitrogen-instead-of-air
I like the opener:
Oh, there are plenty of benefits: (1) Cool fluorescent green valve stem caps (assuming your nitrogen vendor has any marketing savvy), which will look sharp with your spinning wheel covers. (2) Bragging rights. OK, you were behind the curve with cell phones, iPods, thong underwear, etc. Nitrogen in tires is relatively new to the mass market. Now's your chance. (3) Reduced fire danger next time you land your space shuttle or commercial aircraft, and tell me you won't sleep better knowing that.