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Rim diameter
 
Although there are several sizes to choose from, the choice has to be made clear. The debate over large wheels/rims has been a heated one. Even though popularity of the larger rim has increased, the sensability on useage has not. Since their entry into the truck market, functionality has decreased. Granted, the argument of less deflection of the side wall for better handling is a valid one. Performance applications are based upon this. The wider contact patch, combined with thinner sidewall, translates into better traction. I am here to help decipher misconceptions. That formula does not hold true on an off-highway vehicle. In extreme terrain, the sidewall does not deflect as much as needed. A larger sidewall area translates into a more flexible surface. The larger that surface, the less likely a cut. When encountering a rock, the tire virtually wraps itself around it. This is where the traction is derrived from. If a larger rim, and smaller sidewall are used, then the sidewall tends to be pinched between the rock and rim. Not only does this destroy the tire (and they're expensive), but also the rim. This does not mean only buy 15's. This is to state that there should be proportions to consider. For a rig that is used daily, it's a simple formula. Ideally, you want a smooth ride. An extremely large tire, with a tiny rim, has too much deflection. This causes massive vibration on the street. Here is the simplest formula: Overall tire diamter/2 = Rim size. Ideal situation is the rim be half the size of the tire. One can go one size larger or smaller. This will primarily maintain factory ride and handling. Example: 32" tire. Ideal rim: 15", 16", or 17". So if you're set on those 20's, and want to run off-road.... Your best bet is a 38"-42" tire. Your rig will handle like it was designed, while increasing ground clearance.