Parts of Europeans cars are pretty much always in metric: 10 mm nut, 15 mm diameter hose and so on. After all, most of the world uses metres, centimetres, kilograms, watts, and other Si system units. However, for some reason rims are still measured in inches, regardless of where you live. And that’s a little weird.
For example, your car can use 235/45 R17 tires. The first number is the width of the tire in millimetres. The second is the ratio of width to wall height. The third is the diameter of the rim in inches. Why inches? Why even in countries that measure everything in millimetres and centimetres the diameter of rims is described in inches?
In order to answer this question we will have to dive into history. First of all, let’s take a look at when some of the biggest tire manufacturers were founded, because the diameter of the rim is basically only matters for the tires. Michelin was founded in 1889, Dunlop – also in 1889, Pirelli – in 1872 (but did not produce tires at first), Goodyear in 1898, Continental in 1871 (starter producing tires in 1898). The first pneumatic tire was designed for a bicycle, not a car. And for a long time those tires were glued to the rim. This meant that changing them was difficult and took several hours.
In fact, this is how the Michelin company was started. Brothers Édouard and André Michelin owned an agricultural implement business. Once a cyclist came to their shop with a flat tire. It was glued to the rim, as was usual at that time, so it took three hours to repair it. And they still had to wait for the glue to cure. A few hours later Édouard tested the bike and his newly repaired tire immediately went flat. Michelin brothers decided that it was necessary to create relatively easy-to-change tires. This decision ultimately separated the production of rims and tires, and wheel became a real two-part object.
Jumping forward a few decades we will reach the first mass-produced cars. Of course, the very first truly MASS-produced car was the Ford Model T, manufactured in 1908-1927. At that time, many people also drove Rambler, Oldsmobile, Cadillac, Hudson cars. The Austin 7, produced between 1923 and 1939, is often referred to as the British Model T because it was also very popular and became the first car for many families. What do all these automakers have in common? Brits and Americans measure stuff in inches. Thus, tire manufacturers in both Europe and the United States mainly developed tires to fit rims that were categorised by their size in inches.
Tires are quickly wearing parts. At the same time, they were easy to standardise, because tire manufacturers were older than largest car producers. And natural rubber was an entirely separate industry. The biggest and most important tire manufacturers had to adapt to the market conditions, and this did not bother the drivers, because they also had an interest in making tires from Ford fit on, for example, Peugeot.
The influence of the Detroit giants on the global car market did not fade for a long time and the measurement of rim diameter in inches became the standard. Inches in this case actually have the advantage of being a larger unit (1 inch = 2.54 cm). Want 15, 16 or 17 inch rims? A difference of one inch is noticeable and practical in this case. Measuring in centimetres would be more confusing, you would have to skip a lot of numbers in the scale. For the same reasons – global standards and the size of the units – the displays are measured in inches as well.
Ok, history, standards, ease of production and avoiding confusion. However, tire width is still measured in millimetres. And everywhere, including the US. This was not always the case – the tire width of the Ford Model T was indicated in inches. Back then it was a less important measurement because all the tires were similarly narrow.
Everything changed suddenly in 1946 when Michelin, a French company, introduced the Michelin X – the first mass-produced radial tire. It was a huge innovation – more stable, stronger and safer tires meant a lot. At that time manufacturers of cars and rims were already everywhere, everyone needed tires, and sticking to an inch standard just for the sake of the US didn’t make much sense. Therefore, Michelin radial tires were described in both inches (rim diameter), because people were used to it, and millimetres (tire width), because Europe, baby!
Michelin grew rapidly and radial tires spread throughout Europe and Asia – lands that were familiar with millimetres. The Detroit giants, not wanting to acknowledge European innovation, initially avoided radial tires. Only in 1970, 24 years after the introduction of the Michelin X, Ford presented their first car with standard radial tires – the Continental Mark III was equipped with Michelin tires. The US auto market quickly adapted, but the late adoption of radial tires prevented Americans from changing the standards that had been established through more than two decades.
To sum up, originally the diameter of the rims were measured in inches, because American cars dominated the world. The width of the tires is measured in millimetres, because eventually European tires became widespread and didn’t focus on the US exclusively. This is how such a mixed description of tires was created.