How to Read a Rim Sizeby Jody L. Campbell
Most rims come with three common sizes stamped or cast into the back side of the hub. These are the most important sizes to determine if the rim will fit the tire and the vehicle you want to put it on. However, there are two other measurements that are often disregarded, and these can be equally important to fitting the tire and the vehicle properly.
Locate the stamped size on the back of the hub on the rim. This is the assigned measured size of the rim, and it is broken into three sizes: diameter, width and bolt pattern. For example, a 14 by 6 by 4.5 reading on the rim indicates that the diameter of the rim is 14 inches, and it will accommodate most 14-inch tires. The 6 in the middle indicates the width of the rim. Tires have flexible sidewalls and can fit to most rims, but it's best and safest to use a width chart so as not to compromise a tire or have it fail because it's too wide for the rim. The 4.5 is the bolt pattern, measured from the two bolt holes farthest apart. This number also determines whether the rim will fit on your vehicle or not.
Measure the rim width and diameter with a measuring tape if you cannot locate the rim size on the interior hub face of the rim. The diameter is measured from the bottom of the bead on one side of the back of the rim to the other side of the back of the rim. The width is measured from the bottom of the bead on the back side of the rim to the bottom of the bead on the front side of the rim. Measure the farthest distance between the bolt holes to determine the bolt pattern.
Write the numbers down on a piece of paper.
The backspace and offset of the rim are also important measurements to determine whether the rim will work with the tire and vehicle application.
Measure the backspace of the rim by running a straightedge across the rim bead and dropping another straightedge down to the face of the wheel. Record that distance on the piece of paper.
Determine the offset by observing in what area the hub of the rim is located. For example, is it completely centered between the front and back rim bead (zero offset)? If not, is it a positive offset (a deeper inset hub extends the tire outward) or a negative offset (a shallower inset hub will sink the tire deeper into the wheel well)? Make a note on the paper whether the rim is zero, positive or negative offset. Some states do not allow a tire's width to extend a wheel well, and a positive offset rim may manipulate the position of the tire to do just that.