Agricultural Tyre Supply & Repair

Understanding Tyre Sizes

12.4 - 28 (12.4 x 28), 18.4R30 (Imperial)

Indicated in inches, imperial tyres display a string of numbers with a letter or dash in the middle. The first digit represents the width of the tyre in inches. The letter R indicates “Radial” and represents the tyre’s construction. The dash or “x” in the indicates the tyre structure is diagonal (Crossply). The final digit represents the rim diameter in inches.

650/85R38, 460/70R24, 600/70R30 (Metric)

Indicated in millimeters, metric tyres are composed of a string of numbers, a slash and letters. The first number indicates the tyre width in millimeters. The second digit after the slash represents the sidewall height in percentage of the width. The larger the figure the higher the profile of the tyre. “R” represents the Radial construction of the tyre. The final digit represents the rim diameter in inches.

16x6.50-8, 23x10.50-12, 15x6.00-6

The most common method of tyre size for turf and ATV tyres. The first part is the overall height (from the ground to the top of the tyre) of the tyre in inches, the second part is the width of the tyre in inches, and the third part is the rim diameter in inches. For example a 25x10.00-12 ATV tyre would be 25" tall, 10" wide and fitted to a 12" rim.

4.00-8, 5.00-10

Common sizes on old trailers and farm machinery. The first part 4.00 is the width of the tyre and height of the sidewall in inches. The -8 is the rim diameter in inches. A 4.00-8 tyre would fit a 8" diameter rim, be 4" wide and 4" tall on the sidewall. 

4.10/3.50-4, 4.80/4.00-8

Common sizes on sack and wheel barrows. The first, 4.10, is the approximate width of the tyre when fitted to the recommended wheel. The second, 3.50, is the height of the sidewall, and the third is the rim diameter.

Tyre Construction. Crossply vs Radial

Radial (R)

The radial structure is composed of a flexible casing build with several casing plies, as well as several stabilization plies placed on tread area to make it flat and provide lesser tread deformation. The more flexible sidewalls therefore work independently to the tread which is strengthened.

Radial tyre provides equal ground pressure by dispersing load to the surface. It prevents better soil compaction, improve traction and contributes to time and fuel savings.

Cross-Ply

The Cross-Ply structure is composed of an assembly of criss-crossing plies which form a thick layer which is sensitive to heating and less flexible than the radial structure.

The Cross-Ply tread will appear rounded and deeper than Radial meaning the grounding surface is narrower, leading to the concentration of the load to the centre. It will increase compaction.

Speed Rating

The Speed Index is defined as the maximum speed authorised for the tyre when loaded. It is the ratio of the tyre’s load capacity to the vehicles forward speed.

The speed index is determined by the manufacturer. It is indicated on the tyre’s sidewall, just after the load index, in the form of an alphabetical code with or without a number.

Load Rating

Each tyre is designed to carry a maximum load defined by the manufacturer. You can find the load index written as a code on the sidewall.

This is a reference value of the load that a tyre can carry, in relation to a standard pressure of 1.6 bar and a given speed.