The stock FJ Cruiser tires are a very good size for most situations, it's very rare that you'll need tires larger than 32” in diameter. However, there are very good reasons to upgrade the size of your tires. Larger tires increase ground clearance, provide more traction, and enhance the look of your vehicle. They'll help you crawl over and through more difficult obstacles, and certainly give your FJ that 'cool' look. These benefits do, however, come at a cost. When upgrading to 33” or larger tires, your gas mileage will certainly suffer. The cost of tires goes up dramatically with the larger sizes as well.
We could write an entire issue of FJC Magazine covering the different wheel options available. Alloys, steelies, bead lockers, and custom wheels are a very complicated subject to discuss. When choosing an aftermarket wheel, you have to make sure details like offset, backspacing, and vehicle clearance are all within specs. Once you've found wheels that will fit, you have to decide which ones will look the best with your FJ. Tires, lift kits, and other mods will all affect which wheel you'll choose. For more info on wheel dynamics, visit this thread onf FJCruiserForums.com:
Another topic that can be confusing to individuals new to off roading is tire pressure. Maintaining a proper street pressure is just as important as airing down when on the trail. When you upgrade to A/T or M/T tires, this importance is amplified. Pressure that is too high or low will cause your tires to wear unevenly and will need replaced more often. Proper pressure also ensures that your suspension and drivetrain are working properly, and keep your gas mileage up.
By lowering your tire pressure, or airing down, on the trail allows your tires to conform to terrain, resulting greater traction. Lower pressure also helps to absorb some of the bumps on very rough roads.
Choosing the right tire and wheels for your FJ will not be an easy process. How you use your truck, how often you'll be off road, and your climate will all determine the solution that's best for you. Hopefully we've given you some information to help in your decision.