What is car tyre size and what do the numbers in tyre sizes mean
With the help of the following pictures we will explain what the tyre size numbers mean.
The first image here shows the car Tyre size numbering on the side wall of a car Tyre and it reads '205/65 R15'. We shall explain what these car tyre size numbers mean with the help of the second image at the bottom showing the physical dimensions of a car tyre
The first number 205 in the car Tyre size is the tyre width ('W' in the bottom image). The second number of the car tyre size, after the slash, is 65. This is a ratio between the height and width of the tyre expressed as a percentage and is called the Aspect Ratio of the tyre.
What is Aspect Ratio of car tyres
Aspect ratio can be defined as the height ('H' in bottom image) of the tyre divided by the width ('W' in bottom image) and then multiplied by 100. So in our example 205mm is the width of the tyre and the aspect ratio being 65, means that the height of the car tyre is 65% of the tyre width. Thus the height can be calculated as 205x0.65=133mm. The lower aspect ratio of a car tyre, or the second number in a car tyre size, the wider the tyre will seem, that is the width of the tyre remains the same, but the section height of the car tyre size reduces, making the tyre look fatter or wider. In our example if instead of 65, the aspect ratio was 50, then the tyre would still be 205mm wide, but the height will only be 50% or 102.5mm.
The letter 'R' in the tyre size means it is a radial tyre. Now a days almost all cars are fitted with Radial tubeless tyres. In the older days people used to ask for radial tyres and make sure that this letter 'R' was there on the tyres. But now a days the manufacture of older versions of non-radial tyres for cars have been almost completely phased out in India.
The last figure is the Rim diameter or the inside diameter of the tyre in inches. You may wonder why all other tyre dimensions are in millimeter and why only the rim diameter is in inches. This so happened because the Rim sizes are standardised in inches and you get 12, 13 14, 15, 16, and higher inch rims.
There will usually be other markings on a car Tyre to represent the load ratings and the speed rating of tyres. They are important of course, but not much attention need to be paid to those numbers because there are strict Indian standards on car tyre load rating and speed rating of tyres being sold in India. The load rating of Tyres are of more importance to commercial vehicles which carry loads and the speed rating is of interest for car racing drivers.