A guy asked me a while ago how to read the tire size codes in a wheel. After explaining him, he complained because his results did not match a measurement made on a real wheel. I´ll try to explain why:
205/55R16. What does this mean?
Well, don´t tell me why they did it, but the guys that decided how to code tire sizes made a complete mess around something, in theory, really simple: width, sidewall height and rim diameter. That´s all, but expressed in the most strange and disturbing units they were able to find. It could only be stranger if expressed tire width in light-years…
Tire width: expressed in millimeters. Everything fine by now. Just measure your tire´s width, from a top view, when no load is applied to it.
Tire sidewall height: expressed as a percent of the tire width. So, if the width is 205 mm, a 55% of that (the sidewall height) is 112.75 mm.
Rim diameter: expressed in inches. To translate:
- inches to meters -> multiply by 0.0254
- meters to inches –> divide by 0.0254
So, the rim´s diameter will be 406 mm in this case.
An easy calculation: wheel (tire+rim) diameter
If no stress is applied to the wheel, the diameter should be:
|Wheel diameter = rim diameter + (sidewall height x 2)|
It is important to note that the sidewall height should be multiplied by 2, as it´s the height of one side only (see next picture).
Why do I get different results if I measure my tire?
When you take a tape and go out to measure your car´s tires, you will see different results. Specifically in the tire´s sidewall and rim proportion. You should not find any differences in the wheel diameter or radius. Why´s that? Easy… because you are not doing it properly, and this mistake is seen in many places.
The rim diameter should be measured from the inner part where that the tire bead sits on, not from the exterior visible part. So, in order to properly measure the rim size, you will have to take the tire off the rim first, or make an estimation of the amount of tire sidewall that fits inside the rim (usually a bit more than 2 cm, or a bit less that 1 inch).
So, measured from the exterior part, with the wheel mounted, we will get at rim (more or less) 2 cm bigger (and of course, 2 cm less of sidewall height).
To know more about tires: The wheel and tire bible