Difference in gear ratios can be found on Mazdatrix site FAQ, see ref:
http://www.mazdatrix.com/faq/ratios.htm
It's a 3.90 (auto) vs. 4.10 (all manuals except GTU-S, which is 4.30). Don't know what that would translate to in a MPG improvement, if any. It may well hurt your accelleration enough where you'll find youself flogging it just to get out of your own way, which would likely make your MPGs worse. Especially if you have hills to deal with. IMHO, don't bother - you would be better off picking a beater econobox as a daily driver.