i believe that if gears accompany a tire/rim upgrade, they often may cancel each other out. ie- 16in with 3.23 gears to 18 in rims with 3.73 gears is not much a difference. check out this site
www.tchracing.com/tech.php and select the gear calculator. also check out
http://www.precisiongear.com/pgtech_tire.htm to see what a tire change can do to gear ratios.