Check out this 'gear calculator':
http://www.4lo.com/calc/geartire.php
My 05 had 4.10s stock, when I move to 37s it started to drag ass, so punched my info in the 'calculator' to find out what gears would give me an effective ratio closest to stock.
The tire size change had reduced my effective ratio to 3.88 and the calculator indicated that 4.33s would put my effective ratio back up to the stock level. Since they don't make 4.33s I went to the next 'lowest,' 4.56s, and have been a happy camper every since.