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We all know what things like 255/45 19" mean on a tire.
But let's only focus on the first and the last number (the 45 is a percentage, so it has nothing to do with Metric or English)

255 = the width of the tire in millimeters. So the width of the tire is expressed in the Metric System.
19" = the outer diameter of the rim, expressed in inches, or in other words in the Imperial System.

Does anybody know why they mixed these two systems?
 

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Historically tire width has also been specified in inches. However in the sixties the design of the outer part of the tires changed quite a bit, see radial tires.

These were mainly developed/commercialized by Michelin, a French corporation. They developed the specification of the outer part and they probably preferred to give it in the metric system they were accustomed to. Plus the different specification would probably avoid confusion between the different tire types.
 
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