There are several critical measurements to take into account when selecting a wheel or when replacing your stock wheels with aftermarket wheels. Among these important measurements are:
This is the total wheel diameter when measured from bead seat to bead seat. When replacing tires you of course must know the wheel diameter, but this will be stamped on the old tires. If you are browsing through a stack of wheels that are not stamped an easy way to determine the diameter is to measure the maximum diameter from rim to rim and then with a ruler measure the distance from the rim down to the bead seat. Subtract twice the distance which you measured down to the bead seat from the rim to rim diameter and you will have the correct wheel diameter. Wheel diameter is a critical consideration when changing or altering brake components. It is also believed by many that 15" and 16" wheels have a superior bead design and better bead retention than 16.5" or 17" wheels due to a raised inner ridge or "safety bead" that helps to keep the tire bead in place.
The wheel width or rim width is the distance between the outside edges of the bead seat. The most popular rim widths for 4x4s are 6", 7", 7.5", 8" and 10". Rims wider than this are usually only used on trucks with significantly wider tires than are commonly used on most people's rigs. The tire manufacturer will publish guidelines for what rim width should be used with which tire. In general 7" rims will comfortably work with tires up to 10.5" wide, 8" wide rims work for 9.5" up to 12.5" wide tires and 10" wide rims are used for 12.5" and wider (up to about 15" wide). Many popular rim styles may not be available in widths wider than 8" and rims wider than 10" are much more difficult to find and will genrally command a premium price.
Rim width relative to the tire's section width has an effect on bead retention when aired down, and on the overall tire profile. In general a narrower wheel (from within the manufacturers guidelines) will result in a more rounded profile with a slightly smaller footprint but with a somewhat greater degree of protection for the wheel. Narrower wheels also may retain the bead marginally better when aired down than will a wider wheels. Wider wheels provide a flatter tire profile and wider footprint. Since the rim is wider it will not be as protected from the rocks as would a narrower wheel with the same tire (due to the buldge of the tire). The tire carcass will exert less pressure on the wider width bead when aired down and a wider wheel may thus be marginally more likely to lose a bead when pressures are low. Wider wheels, with less sidewall bulge, may reduce the rubbing of large tires on the springs at full turn.
Backspacing is the distance from the inside rim surface to the backside of the wheel mounting surface. It determines how far a wheel (& tire) sticks into or out of the wheel well of the vehicle. Wheels with a lot of backspacing will stick further in. Wheels with little backspacing will stick further out. Sometimes when swapping in wider axles 4x4 owners will switch to a wheel with much more backspacing to compensate. You should note that the backspacing measurement is critical when considering the clearance of the wheels & tires relative to the suspension, braking, and steering components as well as to the body. All of these must be considered in both normal conditions and when at full suspension travel or articulation.
Offset is the distance from the exact wheel centerline to the inside wheel mounting surface. Offset and Backspacing are related. A large amount of offset can change the leverage and the loads on axle or wheel bearings and so, if possible, you should stay as close as possible to the stock wheel offset. In addition to changing the load on the bearings, changing the offset significantly will also change the turning radius (of the tire) and may affect both steering response and steering stability.
The bolt pattern has two essential parts. The first is simple it is the number of bolts. The second is the diameter of the "bolt circle". The bolt circle is an imaginary circle which passes through the centerline of each of the hubs wheel fasteners. On a wheel with 4, 6 or 8 bolt holes the measurement is simple: Just measure from the center of one hole to the center of the hole directly across from it. On a wheel with 5 bolt holes this is not possible. The next best thing, which will provide a "close enough" approximation is to measure from the center of one bolt hole to a line (or ruler) that is drawn between the opposite edge of the two opposing bolt holes. The bolt pattern is then indicated by the number of bolts X the bolt circle diameter - for example a 5x5.5" bolt pattern is 5 bolt holes arranged around a circle which is 5.5" in diameter. This may also be commonly referred to as 5 on 5.5 bolt pattern.