You probably expected a historical introduction at this point, but standardisation plays such a crucial role in tyres as in nearly no other part of the motor vehicle that the assumption is probably justified because many users of this book also use it for reference. Without any further ado we will start right away.

Here the wheel in an unusual perspective. No, it is not completely cut in the middle, the tire yes, the rim no. The latter has a drop-centre, which makes it even possible to mount the tyre. The seat of the tyre is between the drop-centre and the protruding edge (rim flange) after the installation is complete.

This is also where the dimension d_R comes into play, which of course must be the same for both tyre and rim. So if the tire has the designation 205/55 R 16, then this could be the same for the rim 6J x 16 where this rim or inner tyre diameter is expressed in inches. Millimetres are obtained by multiplying by 25.4, which in this case gives about 406 mm.

The 6 in the specification of the rim stands for the so-called rim width, a term that describes very well the distance between the two rim flanges. It is not standardized, but it must fit to some extent. A 255/55 R 16 tyre on a 4J x 16 rim would look unusually constricted and a 155/55 R 16 tyre would be unmountably widened to 8J x 16.

You will notice that the first number in the tyre designation indicates the width of the tyre. And so that we don't get too used to it, this time in millimeters. But what is the width? The picture above shows it, but leaves questions. Naturally the tyre would be wider on a wider rim. That is why the rim to be assigned to each tyre when measuring the width is standardised.

However, if you check each size at a tyre shop, you will always find a '5' at the end of the first number. We speak of so-called grid dimensions. All tyres from 250 to 259 mm width are given the code '255'. This simplifies stockholding, which is becoming increasingly complex anyway.

And what about the recurring '55' behind the slash? This indicates the height-width ratio. So the 255 tire would be about 255 mm times 0.55 equals about 140 mm high. This ratio is currently reaching incredible values. Probably at 0.25 one end of the scale has been reached, because at values below that the tyre is probably no longer mountable.

Now you should be able to determine the height of the tyre with the designation 255/55 R 16. But you have to take the 140 mm twice, because a wheel consists of tires on top and bottom. The 406 mm rim diameter makes 686 mm.

This is a very theoretical value, because of course the tyre is clearly pressed being mounted at the vehicle. If we take half of it with 343 mm, then this is still not the value that one would measure from the middle of the wheel hub to the road.

It gets even worse, because the value does not remain constant either. The tyre is inflated by centrifugal forces as the wheel speed increases. This radius becomes larger the faster the vehicle goes. This has been stopped and it has been agreed to 60 km/h. The radius, now called dynamic, is thus determined by the distance travelled and the number of wheel revolutions.