# Car tire dimensions, why oh why 205/55/16.

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Why is a tire’s dimension described using inches, millimeters and percentage? Is it to annoy ppl or to make the believers of the imperial and metric system happy, all at once?

In: 51

It’s basically because early on the US dominated auto manufacturing so wheels got to be measured in inches.

Later on Europe dominated tire manufacturing, cause of the introduction of radial tires from Michelin.

One hitch was added in because the US ended up requiring inch rim sizes in different regulations so to sell in the US tires had to be marked with the inch sizing.

The aspect ratio seems like it’s thrown in to mess with you even more. However the aspect ratio is meant to convey performance changes from wider tires. This doesn’t scale with an absolute width though. The same width in a small tire performs different than that of a big tire. So we get a ratio instead.

And so now we have different measurements and no one really wants to bother having two systems in place for like a decade of transition to make it more consistent.

It’s not only car tyres. 20″ Bicycle tyres? Actually 16″, they’ve decided to do the outer rolling diameter for measurements there…but on moto tyres? It’s rim diameter like on your car.

You could make an argument for why we use inches for the diameter but while we’re measuring tyre width it’s millimeters but when it’s a rim width? Yup back to inches. Put that 265mm tyre on that 9.5″ wide rim

Tyre manufacturers are sadists, you’d probably need another ELI5 to explain it.

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What do you mean by percentage? The numbers are mm, mm, inches.

The first number (205) is the diameter of the width of the tire in millimetres, the second number (55) is the lip of the tire in millimetres ( the rubber on the sides) and the third number (16) is the diameter of the rim in inches.