Which Wheel Where
A new set of wheels is a big purchase for any Jeeper. On an editor's budget, it means a month or two of eating Ramen Noodles and often works out to costing us more than buying a new (used) Jeep. The problem is that there are so many choices out there-from wheel size to wheel material to wheel construction-it is easy to put a foot wrong. It's really hard to pick wheels without seeing them on your Jeep, and we've had our fair share of gaffes over the years. The key is to sit back and consider the options to make an informed decision. We'll leave the looks of the wheels out of this, and instead will help you out with the construction and material choices.
A general rule of thumb about wheel size is that the bigger you go the bigger brakes you can clear, the heavier the overall package becomes, but the less sidewall you have. We look for as much sidewall as the wheel diameter, so on a 15-inch wheel, we'd want the tire to be at least 15 inches bigger for a 30-inch tire height. Of course, you can't always have your cake and eat it too, but that basic rule holds true for all wheel sizes. A 16-inch wheel means a 32-inch tire minimum; a 17-inch, 34-inch tires; and an 18-inch, 36 inches of rubber. We find that if we play in rocks and pick a shorter tire, we end up grinding the wheels more than we'd really like to. This guideline also saves the rims from curbs when the old lady takes the Jeep shopping.
The width of the rim and the backspacing are the most often discussed sizes, and we still find that tire manufacturers recommend too wide a rim for a particular tire size or too little backspacing for a typical situation.
Most manufacturers won't recommend an 8-inch rim for a tire with a 12.50-inch overall width. The reason for that is there will be more sidewall roll on-road if you use an 8-inch rim, which could lead to stability problems if you drive your Jeep like Mario Andretti. The problem for us is that the 10-inch-wide rim just won't hold the bead as well down in the single-digit air pressures. So, we tend to look at a rim that is 4 inches narrower than the overall width for the best performance at lower pressures. A 14.50-inch-wide tire would then go on a 10-inch rim, and so forth. The only exception would be if we were going for a particular look. For example, if we wanted the wide and square look and we were willing to forgo some off-road ability to get it.
Backspacing ties into rim width as well, and both affect your scrub radius. The further the center of the tire moves away from the centerline of the ball joints the more scrub radius you will have and that leads to worse on-road handling. The other thing that will happen is that with less backspacing more force is exerted on the ball joints, wheel bearings, and tie-rod ends which leads to earlier failure of these components. A wider rim with the same backspacing will move the mass of the tire out from the ball joint center line more than a narrow rim. Remember, "less" backspacing means a larger number, which means the tire is sucked further underneath the Jeep. We try to run the largest backspacing we can that will still clear our suspension, brakes, and inner wheelwells for a given tire size. If you aren't sure, take off your existing tire and rim and determine backspacing, rim width, and tread and section width. Then with some simple math you can use a combination of blocks of wood, duct tape, and maybe even wheel spacers to determine where the tire and wheel would end up with the new rim.