Fixed ratios

In general, a wide range of tyre markings are encountered on tyres. One of these markings is the height/width ratio, which is always found on the side of the tyre. How is it calculated?

Height divided by width

The height/width ratio is calculated by comparing the height of the tyre with its width. Standard tyres have a ratio of 1, and are part of the100 series (= height and width are identical, which means height divided by width is 1). In this case, the value is displayed in inches and whole numbers (e.g. 18.00R33). An inch is equivalent to 2,54 cm.
Next in line are the 80 series tyres, where the ratio between height and width is 0,8. Size is indicated in whole, half and quarter inches; for example, 525/80R25.
However, 70 series tyres were also available in several sectors (height x width = 0,7), but were fully incorporated into the 65 series. In this case, height is 0,65 x width. Sizes are expressed in whole inches or whole millimetres. Examples include the 35/65R33 and the 750/65R25.

Operating costs

Tyres can represent a major investment. Many operating parameters determine performance and whether you make the most of your tyres. Although these parameters are determined by operating and maintenance conditions, the tyres you select must be used for the purpose for which they were designed. The price per kilometre or operating hour is – depending on tyre type and sector – determined by operating conditions and not just by price and quality. As a result, the height/width ratio must be compatible with the intended purpose.