How Are Tire Rims Measured?

There are five measurements that distinguish tire rims. The first is the height of the wheel measured diagonally. Measurements start from the smallest wheel at 12 inches to monster 22-inch wheels and larger, although wheels larger than 22 inches are not as common. The wheel size also determines the tire size---if you have a 16-inch wheel, you will have a 16-inch tire.

The next measurement is the width of the wheel measured across. Wheels measure 4 inches across (mostly "skinny" tires for the purpose of drag racing), 7-inch wheels are the most common, and wide tires measure 10 inches. The next measurement is a matter of offset. Offset is used for placement of the tire under the fender well of the car. For illustrative purposes, think of it like this: The center of the wheel stays in a constant position, resting flat on an object with the wheel a few inches off the floor. Move the outside of the wheel up or down without moving the center. This is offset. By varying the offset, the tire can be moved either out or in as desired, as long as the car will allow it without contacting something. This is usually used on the rear tires for those desiring wider tires than stock.

The stock rim is generally 7 inches and does not allow for a much larger tire. A 10-inch rim should be purchased for a larger tire. A 10-inch wheel with the stock offset would sit with part of the tire outside the wheel well. A different offset is needed to move the tire under the car. The first thing to do is examine the area well. Look for interference from the shocks, frame, shock mounts, curvature of the wheel well, or any other obstruction. Remember that the closer the tire gets to the curve in the fender well, the less up and down movement possible before hitting the tire. The tire always bulges out farther than the wheel, so always measure from the tire. Measure in from the tire to the closest obstruction and allow 7/16 of an inch. Lay the wheel on a flat surface and measure the distance from the floor to the bottom side of the center of the wheel. If it has been determined that the tire can be moved in 2 inches for example, add 2 inches to the number of the offset. Now consider that there will be a larger wheel used. Going to a 10-inch wheel to accommodate the larger tires increases the width by 3 inches.

When purchasing the wheel, ask if the offset is constant and whether the manufacturer moved the wheel out 3 inches or distributed the size difference equally. If the manufacturer distributed the size difference equally, this means that the new wheel will have 1½ inches more depth than stock, so the 1½ inches must be subtracted from the new offset.

The next measurement is the wheel pattern. It is measured diagonally from the center of one wheel stud opening to the farthest one. When asking for wheels, other than just stating the vehicle and letting the tire store do the math, give a good description of the wheel and tire. For example, you may specify that five lug nuts are 4¾ inches apart. The last measurement, although not normally needed, is the center hole diameter. In some cases it is important since the hole may be the wrong size to fit a particular axle.