While the FJ Cruiser is one of the most capable off road vehicles when stock, most come with a significant limiting factor: tires. The Dunlop tires that come with most FJ Cruisers are good for pavement driving and offer decent tread life, but they're not very well suited to most off road situations.
When the time comes to upgrade your tires, the choices are almost limitless. It's not within the scope of this article to cover EVERY tire on the market, but we can talk about the two basic options: All Terrain (A/T) vs. Mud Terrain (M/T). A quick disclaimer, the benefits and drawbacks that we'll talk about for each type of tire are general, and may not apply to EVERY A/T or M/T tire. Please research the specific tire you're interested in before making your decision.
The A/T will be the preferred tire for many people that use their FJ as a daily driver. Traditional A/T's provide a very good mix of on and off road capability. They come in various tread patterns, and most have a tread life of over 30,000 miles. While A/T's are very good off road tires, they're not rock crawling grapplers or mud boggers. All Terrain tires will tend to slick over in deep snow and mud, which will definitely limit their trail use in some climates.
Most M/T tires are easily identifiable by the large lugs in the tread. They're much more aggressive than most A/T's, and provide increased off road performance. Mud Terrains will tend to 'self clean' in deep mud and snow, and provide excellent contact area when aired down and crawling on rocks. If you use youre FJ as a daily driver like I do, there are some definite drawbacks to M/T's. First, many (but not all) M/T's tend to wear out quicker than other types of tire. Mud Terrains also tend to slide around more on ice, and can provide less traction in heavy rain.