Tractor tire sizes are denoted by a standardised system of numbering that provides details of the tyre's width and diameter; in some cases, there is also an indication of the tire height. Understanding this system will help you choose the right tires when it's time to replace the older ones on your tractor. This information is easy to understand from a study of the size that's branded on the tire sidewall.
Locate the code on the tire. The numeric code on your tractor tire may be obscured by mud clinging to the sidewall. Wash the tires thoroughly to be able to see the numbers.
Observe the tire code. On the sidewall of the tire, you'll find a series of numbers separated by a slash or a hyphen. Check if there are two or three numbers branded.
Understand the code. All dimensions of the tire, in terms of inches, are represented as W-D or H/W-D. "W" stands for width, "D" is rim diameter and "H" connotes the percentage height of the tire. For example, if your tire shows the presence of two numerals, the first number denotes the tire width and the second one indicates the rim diameter. If there are three numbers on the sidewall, the first number denotes height of the tire, the second one the width and the third one is the rim diameter. If your tire reads 25/5-15, it indicates a 25-inch high tire, which has a 5 inch width and will fit a 15-inch diameter rim.
In most cases, the height of the tractor tire is maintained around 70 per cent of the width, although a few tire manufacturers may deviate from this norm. If you have an antique tractor that dates back to the 1930s, chances are that your tire numbering system is different from the newer system. The size of the rims should match that of the tire --- if the rim is wider as compared to the tire sidewall, you'll need to exert greater air pressure to achieve good traction.