Deciphering the puzzle of numbers in ascertaining tractor tire size


It often happens that you buy a tractor for some particular purpose of landscaping and in the course of its use, the need changes. In such circumstances, it may turn out that, the tires fitted with the equipment are not exactly what you need for your present requirements. Since the type of tire relates to its functions, it is essential to know about the tire features and sizes for buying replacement tires. The size of the tire is perhaps the most important feature followed by some other aspects like tread and ply. Unless you can determine the right size of tractor tires, you can never buy it right. In this article, we have discussed how you can decode the numbers displayed on tires.

Expressing the size

The tire size depends on the type of tractor and the wheel type. However, you must be able to decode the numbers displayed on the tire to ascertain its size. The manufacturers use two methods of expressing tire size – the standard format and the metric format. In both methods, the numbers appearing in a sequence have a special significance which when correctly understood helps to get the size of tire.

  • Standard format – This is a simpler version of expressing tire size. You find two numbers separated from one another by a hyphen or dash. The first number is the width of the tire in inches and the second number corresponds to the wheel size or its diameter where the tire fits. The dash tells us about the tire construction, as it is bias ply. If instead of the dash there would be ‘R,’ it would mean that it is a radial tire.
  • Metric format – The metric expression of the tire size is a bit complex as it consists of more digits and alphabets. When you come across “525/85R42” the first number corresponds to the tire width expressed in metric units of a millimeter.  Although it is a metric format, the last number is the wheel size or diameter in inches. As earlier “R” is the type of tire and here it is radial.  However, the remaining number in the middle (85) needs some explanation because it denotes the aspect ratio.

Aspect ratio

The number 85 in the middle is a percentage obtained from the ratio between the height of the sidewall of the tire and width of the tire. Instead of expressing the ratio in its original form by using a colon as you find in 5:12, it is expressed regarding percentage. Here 85 mean that the height of the sidewall is 85 percent of the tire width.  From the expression, you can work out the sidewall height as 525 x.85 = 446.25 mm.

More numbers

There may be more numbers used in expressing the tire size as it conveys additional information.  If the number is 520/85-42 158 A8 R1, then the dash tells us that it is a bias ply tire. 158 is load index number, and A8 is speed index number.

The better you are in decoding the numbers easier is the tire selection.