What Wheel To Choose: The selection should be determined by function. Rim diameter and rim width are dictated by the load to be carried and clearance for the brake assemblies and suspension. Anything larger is chosen for the sake of style. The current styling trend toward oversized wheels comes with a number of adverse side effects. The wheel rim is the heaviest part of the wheel and the larger the diameter the more material that is required to produce it. The larger diameter also necessitates a larger diameter center section. When the wheel rotates the heavier rim that is farther from the center of the wheel creates more inertia that has an adverse effect on braking. The heavier wheel also increases unsprung weight that adversely effects ride. See: What Tire to Choose.
Rim Width & Diameter: Rim width is determined by measuring between the bead seating surfaces, not the outside edge of the rim flange. Rim diameter is determined by measuring the inside diameter just inside the bead mounting flange, not the rim flange.
Wheel Offset: Offset, sometimes referred to as "back" or "rear spacing", is the measured difference between the wheel’s mounting face where it bolts up to the hub and the centerline of the rim. When the mounting face directly aligns with the wheel’s centerline, the wheel has zero offset. When the back of the bolt pad is closer to the drum side of the wheel it’s called negative offset. When the back of the bolt pad is closer to the street side of the wheel it’s called positive offset.
Measuring Wheel Backspacing: Measure the rear spacing of the wheel, by laying it face down on a flat surface. Then lay a straightedge across the inside edge of the wheel. Measure from the wheel mounting face with a ruler, and record the distance.
Measuring Wheel Offset: Using the wheel backspacing dimension (A), measure the total wheel width (B) and calculate the offset using the following formula: Rear Spacing — Total Wheel Width ÷ 2 = Offset.
Bolt Circle: The diameter of an imaginary circle through the center of each lug nut hole. To determine the diameter of the bolt circle on a 4,6, or 8 lug nut circle measure from the center of one lug nut hole to the center of the opposite lug nut hole on the other side of the circle. For a 5 lug nut circle measure from the center of one lug nut hole to the center of the lug nut hole farthest away on the other side of the circle and add 1/4-inch. I.e. 5 on 4-1/2 measures 4-1/4" plus 1/4" = 5 on 4-1/2.