Changing seasons can signal a time to change the tires on your car, truck or tractor, and understanding all the letters and numbers printed on your new tires is important – not only so you are satisfied with your purchase but also so you are safe.

Since 1991, tires have been stamped with their size, speed rating, load index and service description. To the uninitiated, a tire size can appear to be a math equation but, again, understanding the meaning and intent of each piece of the tire puzzle can help you get the best fit for your needs.

Let’s start by jotting down a few examples of tire sizes we might have on our vehicles. Our imaginary car uses P195/60R15 tires. Our truck needs LT265/75R18 tires, and our tractor wears 528/85R42 158 A8 R1 on the rear and 480/70R30 on the front.

The first letter indicates the tire’s primary use. Passenger car and lighter-use vehicles generally use tires beginning with the letter “P”. Truck tires (for three-quarter-ton and heavier GVW vehicles) carry the “LT” brand. The “LT” tires are built more heavily and, therefore, carry a heftier price tag as well.

Note: Tractor tires have no need for that distinction. I suppose while it might be fairly easy to fit the incorrect tire on a truck that was intended for an SUV, tractor tires generally don’t fit farm trucks … at least not ones used as daily drivers.

Next is the tire’s overall width in millimeters. Our car tire is 195 millimeters wide. The truck tire is 265 millimeters, and the tractor is 528 millimeters. Of course, these are all metric measurements. To convert them to our conventional English system, just remember there are 25.4 millimeters in an inch. Therefore, our car, truck and tractor tires are 7.7 inches, 10.4 inches and 20.8 inches (rear) and 18.9 (front) inches wide, respectively.

The next number represents the ratio of the height of the sidewall in respect to the tire’s width. The higher the number, the taller the sidewall is in relation to the width. For example, our car tire’s sidewall is about 4.6 inches tall (195 x 0.6 / 25.4), while our rear tractor tire sidewall is 17.7 inches tall (528 x 0.85 / 25.4). This may be a consideration depending on the application of vehicle and how you drive personally.

Low-profile tires are all the rage on passenger cars, but they definitely carry some drawbacks. Since their sidewalls are so short (some are almost nonexistent), they offer no flex when driving into a curve aggressively. This lack of flex makes them less safe in this condition because they are more likely to lose tread contact with the road.

The letter “R” is pretty straightforward. R is for radial construction and, as I have stated in previous articles, I am a fan of radial tractor tires.

The final number is kind of interesting when you note it is an English number in an otherwise metric system. The final number is the height of the wheel (rim) the tire fits.

Diving into the weeds a bit, there are factors when selecting a tractor tire that may be of interest to you. These include the group size, load index, speed symbol and tread design.

Tractor tires are placed into groups by their rolling circumference index, commonly referred to as group size. Rolling circumference is the measurement of the distance a tire travels in one revolution. Tires are given the same rolling circumference index number or group size designation (regardless of tire width or rim diameter) if their rolling circumferences are similar. For example, a Group 42 tire has an overall diameter of 59 inches, while a Group 49 tire’s diameter is 85.5 inches.

The load index is a numerical listing where a greater number equates to a greater load-bearing capacity. The load index presents us with a uniform method to report the load-carrying capacity of a tire. For example, 157 load index means the tire has a maximum load-carrying capacity of 4,125 kilograms (9,100 pounds) at the speed specified by the speed symbol when the tire is inflated to its rated inflation pressure.

When a tire is used in single application, there would be 8,250-kilogram (18,200-pound) carrying capacity for the axle (4,125 kilograms x two tires or 9,100 pounds x two tires).

The load-carrying capacity per tire is reduced by 12 percent when the tires are used in dual application, so there would be 14,520-kilogram (32,030-pound) carrying capacity for the axle (4,125 x 0.88 x 4 tires or 9,100 x 0.88 x 4 tires). When comparing tires, the higher the load index number, the higher the load capacity.

The speed symbol is the top speed a tire is designed to travel. A8 is rated for 40 kilometers per hour (25 mph). B is rated for 50 kilometers per hour (31 mph). D is rated for 65 kilometers per hour (40 mph). The speed symbol designation alone does not determine suitability for use on 50 kilometer-per-hour tractors.

Finally, the tread designation is used to describe the tread and indicate tire usage. Designs offered are all lug- or bar-type tires and are separated into one of three specifications: R1, R1W or R2. R1 is a standard tread and is used primarily for general dryland farming. These tires have the shortest lug height and the narrowest spacing between lugs.

R1W is a wet traction tread for wet, sticky soil conditions. This tread fills the gap between R1 and R2 tires, having a deeper lug with wider spacing than R1 tires but shorter and narrower than R2. R1W is defined as having a lug height about 20 percent deeper than an equivalent R1 tire, but this could vary from 15 percent to 35 percent depending on the tire and manufacturer.

R2 is a tread type used typically with cane and rice or other crops grown in wet muck or flooded fields. Tread depth of R2 tires is approximately twice as deep as R1 tires. R2 tires also have the widest spacing between lugs to allow mud to shed easier. The wide-spaced lugs can show extra wear and cause problems with vibration when used on the road.

R2 tires may not pull as well as R1 or R1W tires in drier soil conditions found in most row-crop applications. Typically, tires with R2 tread should be matched on the front and rear of a tractor, while R1 and R1W treads can be mixed or matched on the same tractor to meet requirements or preference.

No matter what your tire choice or preference is, always operate your machinery in a safe manner. Outlive whatever tire selection you make.  

Andy Overbay holds a Ph.D. in ag education and has more than 40 years of hands-on dairy and farming experience.