Agricultural Tire Size Explanation

Tractor tires come in a variety of sizes and their sidewall codes are similar to the codes used on the tires of passenger cars. The combination of numbers and letters are measurements and abbreviations for tire features.

Interpreting the Code

The numbers on tires may indicate either millimeters or inches depending on where they are made. The first number is the tire section width which is usually written in millimeters; then there is a slash followed by the aspect ratio -- the sidewall height as compared to the tread width -- shown as a percentage. Next you'll probably have the letter R which means radial. If your code does not have an R it is a bias tire. The next number is the rim size, usually in inches, followed by a space, followed by a number that refers to load capacity. The last number will follow a letter A through G; this is the speed rating for the tire.

If You're Seeing Stars

Each star is an additional 6 psi.

Some farm tires also have stars which refer to inflation pressures. The maximum inflation for one star is 18 psi; two stars is 24 psi; and 3 stars is 30 psi. These pressures apply to radial tires no matter what their size or load rating -- the inflation pressure per square inch remains the same.

Match Front and Rear Tires

Coordinate front and rear sizes.

If your tractor dates to the 1960s to 1980s time period, you may have a mechanical front drive tractor. These tractors require very specific pairings between the smaller front tires and the larger rear tires to operate properly. There is a standard chart available for determining the rolling circumference pairings for these tractor tires.

References

• Choosing the Right Tire
• Titan Tire

Resources

• Titan Tire