Why are tyre widths measured in millimetres and the wheels in inches?
You can thank the radial revolution for this. Prior to the development of radial tyre construction by Michelin in the Forties, the major manufacturers were American and British, and they worked in imperial measurements. Michelin chose the metric system for the new technology partly because it had used it previously, but also to distinguish it from the existing cross-ply construction. As the firm’s hi-tech tyres went on existing rims, the imperial sizes remained for this reason. But wheels which use new technology such as the PAX run-flat system are measured in millimetres.