1. Lay the cap face down on the paper.
2. Hold the pencil straight up and mark the paper at the far left and far right edge of the cap.
3. Lift the cap away and measure from mark to mark.

Start at the last whole number, then count the little lines past it. Each one adds 1/16 in. Example: 5 and 11 little lines is 5 11/16 in, our 145mm size.

Flip the cap over. Measure the black plastic clip ring (the raised ring the metal clip sits on), outside edge to outside edge, straight across the middle.

Don't measure the thin metal clip, the hole inside the ring or the wide outer cap.

Tap your back clip ring size to select it.

Factory Rolls-Royce wheels are different: their back clip ring measures 64mm (about 2 1/2 in). Choose 64mm on our Rolls-Royce XL caps.