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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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.
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