Center caps have been around since the days of the horse drawn carriage. Center caps serve a dual role when placed onto the wheel of a vehicle today. The primary function that most people see is the visual effect that centre caps have. The second and most important purpose of the centre cap is to block harmful debris, dirt, and road grime from getting onto and damaging the centre of an axle and wheel hub. If a wheel is missing a centre cap, the enclosed axle hub is prone to excess wear and damage.
Place the centre cap on a table or solid work surface, so that the opening of the centre cap is facing upward. There are three measurements to take on a centre cap for proper fixture on a wheel and vehicle. The inner diameter gives the measurement needed to fit over the wheel hub when the centre cap is on the vehicle. The outer diameter gives the measurement needed to fit the centre cap through the wheel. The depth of the centre cap is manufacturer specific, sometimes for aesthetic purposes, and other times for wheel hubs that extend through the wheel (4x4 vehicles with manual locking hubs).
Place a tape measure or tape ruler across the opening on the centre cap. Measure the inner diameter of the centre cap. The inner diameter is the distance between the two inner walls of the centre cap, measured in a straight line across the centre cap. Mark this measurement on a piece of paper with a pen, as the "I.D" or "Inner Diameter."
Measure the outer diameter at the base or opening of the centre cap, as this is the widest portion of the cap. The outer diameter is a measurement between the two outer walls of the centre cap at its widest point, made in a straight line. Mark this measurement onto a piece of paper with a pen, as the "O.D" or "Outer Diameter."
Hold the centre cap so that the opening is facing downward in your hand. Place a tape measure inside the centre cap until you hit the face of the centre cap, on the inside of the cap. This measurement is the depth measurement, and is the measurement from the inside front of the cap to the base of the cap on the other end made in a straight line. Mark the measurement on a piece of paper with a pen, as "Depth."
Most centre caps today are pressed on from the outside of the wheel or pushed through from the rear of the wheel. There is a large variety of types of centre caps and centre cap mounting methods. Some centre caps have mounting bolts or screws that need to be removed from the front or rear of the wheel, prior to removal of the centre cap. Taking the measurements for all centre cap types is the same procedure.