gears

Hello, I am new to JF along with being a new Jeep owner. I have a 2006 Wrangler X and have been reading posts about the biggest wheel/tire I can install with out a lift. One of the responses indicated not to go over a certain size based on gearing. I have the 6 cylinder but how do I find out what gears (3.07?) I have and what rear end.



Thanks!