Bridgestone Team

Tractors and implements need the right tire sizes, constructions, and tread patterns to help you get the job done smoothly and efficiently. Understanding how to measure the tractor tire sizes and the meaning of all the numbers and letters will help you have a more informed conversation when you are talking to your trusted tire dealer. Here’s what you need to know. 

If the tire size isn’t legible on the sidewall, you can easily measure the tire by hand. The two numbers you need are the section width and the diameter. Before measuring, take all the weight off the tire and inflate it to its rated pressure for accuracy.

• Determine section width: Measure the tire at its widest point, typically found in the sidewall, not the tread. Since it’s tricky to get this measurement with a tape measure alone, a wide pair of calipers comes in handy.  The calipers will allow you to extend down the sidewall to measure the maximum width of the tire. Don’t have calipers? Ask a friend to hold a couple of straight edges, like 2x4s, on each side of the tire past the tread area. Use a tape measure to check the space between the 2x4s—that’s your section width.
• Calculate tire diameter: Measure the circumference at the centerline of the tire. Wrap a string around the tire, mark where it overlaps, and measure the length. Divide the total by 3.14 to find the diameter.

Once you’ve got your tire dimensions, check the Databook for a tire with similar diameter and section width. Your measurements come from a used tire, so new options may not match exactly. Once you’ve landed on a tire size, decide whether you want bias or radial construction—look for an “R” on the sidewall for radial.

The wrong tractor tire size can stand in the way of getting the most from your equipment. On mechanical front wheel drive (MFWD or FWD) tractors, the rear and front tires need to maintain a proper rolling circumference ratio for smooth operation. If that ratio changes, it can put strain on your driveline or shorten the life of your tires.

It’s acceptable to increase or decrease the overall diameter on the rear tires, but the front tire will also need to be changed. On articulated 4WD tractors, if you increase the diameter of tires on the rear axle, the same increase needs to occur on the front.

All Firestone Ag tires include the tire size on the sidewall. The tire size will be either represented in Standard or Metric tire size nomenclature; Check out our  sizing chart: How to read a tractor tire sidewall

While most countries use the metric system, the U.S. sticks with American standard units for tires. That means you may see metric markings on certain tires, and understanding their American equivalents helps you get the right fit. The sizes are also labeled differently than you’ll find on traditional American tires.

• The first number on a metric tire is the width in millimeters.
• The second number shows the sidewall’s height as a percentage of the tire’s width—called the aspect ratio. So for a 285/75R16 tire, the sidewall height is 75% of 285mm (about 11.22 inches).
• The third number, often after an “R” for radial, is the wheel’s diameter.

Use this formula to convert from metric to American standard tire sizing:

Width in inches = section width / 25.4

Section Height in inches = Width in inches × Aspect Ratio (%)

Getting the right tractor tire size maximizes your equipment’s performance and protects your investment. Still have questions or need help with sizing? Reach out to any Firestone Ag Certified Dealer—they’re ready to help. 

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