Eli5: how come tires use marking from different measurement system. Metric for the width, imperial for radius and percentage for height? (like 205/50 R16)
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R16 refers to the 16″ wheel size, not the tire’s radius. The ***/50 is the ratio of the sidewall height to the tread width. A 205/50 has a sidewall height of 102.5mm.
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Everyone seems to be answering what the ratios mean and not what OP asked. I don’t really know either so I’d like to know the real answer myself. But a good guess would be because America uses imperial whole most everybody else uses metric and they’re not going to change all their products for one country. So we have wheel size imperial because that stays common and the consumer knows that number easily, and the aspect ratio of the actual tire gets to stay in metric for the manufacturer.
It’s mostly a bunch of historic accidents based around who was successful in making cars (wheel) and tires.
In the early days of cars America made a lot of cars, this lead to a lot of the standards being established by America. All the numbers on a tire would be in “imperial”. For example a Ford Model A might use a 4.75-19, which means a wheel that is 19″ in diameter, and a tire that is 4.75″ wide. Aspect ratio was probably 90%, which was the default back then.
A lot of the current tire naming technique (called P-metric) is a side effect of European tire makers sticking tires on US-standard wheels. This went on in Europe for a while, but started getting ported back to the US in the late 60’s when Michelin (in France) developed the “radial” tire, which were really good, and created a demand for them in the US. So now you had a metric European tire being put onto a US wheel (as an aside the British had something similar happen in the 50’s and 60’s). The aspect ratio is a historical side effect.
You can also sometimes see “pure metric” tire sizes, like say [this 220/55 R390 tire](https://www.universaltire.com/michelin/michelin-radial-tires/220-55vr390-michelin-trx-b-blackwall-tire.html) which is an effect of European tire makers plus European car/wheels.
You can also find “Pure imperial” tires out there. They’re common in “flotation” tires.
Millimeters are more accurate than inches. It’s tough to represent 8-3/8” without confusion. Fractions are very difficult for a lot of people. That’s why the 1/3 pound burger didn’t win against the 1/4-pounder.
First number is width in mm. Percentage is highth width ratio. And in metric system if you say something in inches. Then you know it is circle diameter. In most of industry.
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