Decoding Tractor Tire Sizes

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Different sized tractors call for different tractor tire sizes, and styles for that matter. Understanding the gibberish printed on the sidewall of your tires is your first step to understanding what you need to buy to properly shod your mechanical workhorse. The second step to picking replacements for your worn out tires is understanding your options and matching those options to your needs, since what your tractor came with from the factory may not be the best tire to do what you need it to do today.

Standard Size Expression

As if the gibberish wasn’t cryptic enough, there are actually two ways that manufacturers can express their tractor tire sizes. One method uses the “standard” format and the other is the “metric” format. The standard method of size expression is relatively simple, but not as descriptive as metric.

An example of tractor tire sizes expressed in the standard format would be “14.9-42”. The “14.9” means the tire is 14.9 inches wide and the “42” tells us that this tire fits a 42-inch diameter wheel. The dash “-” indicates the tire construction is bias-ply, but this could be an “R” if the tire is radial. This example is actually the tire size I have on the rear of my 4×4 diesel tractor.

Metric Size Expression

A metric tractor tire sizes expression may look something like “520/85R42.” The “520” part means the tire is 520mm wide, “R” means it’s a radial tire, and “42” is the rim size expressed in inches, just like the standard method. That wasn’t too hard … but wait, there’s more!

Aspect Ratio

The “85” in the metric example is a ratio, represented as a percentage. Usually, when we write out a ratio, we use a colon to express it like “16:9” which is a common size ratio for computer screens, or “4.10:1” which is a common axle gear ratio, but that’s not how it’s done on tractor tires. “85” means 85 percent, which tells us that the tire’s sidewall height is equal to 85 percent of the tire’s width. A tire that is labeled “520/85R42” has a sidewall height of 442mm (520 x 0.85 = 442). This percentage is called the “aspect ratio” of the tire, which is useful to know when calculating how to properly ballast tractor tires.

Other Numbers

Let’s expand on the Metric example; you may see something like “520/85-42 158 A8 R1” printed on the sidewall of your tire. “520” is still the width, “85” is the aspect ratio, the dash “-” signifies that this is a bias-ply tire, “42” is the diameter of the wheel, but now we have more numbers. “158” is a load index number (which equates to 9,350 pounds), “A8” is a speed index number (A8 means you can go up to 25 MPH with this tire), and “R1” tells you what tread style the tire features (R1 is your standard agricultural cleat tire).

Radial or Bias-Ply

Bias-Ply tires are an old design that has largely been abandoned by the automotive industry. Originally the term “bias-ply” referred to the embedded cotton cloth that was layered atop of each other, placed at a biased angle to each other so the woven cotton threads didn’t all go in the same direction, which made them more resilient than if they were all in line.

“Modern” bias-ply tires use cords that are embedded in the tire, strung side to side and at various angles to each other. Bias-ply tires offer good performance at low speeds and are arguably the tougher tire design, but what is beyond arguing is the fact that these tires cost less to produce than their high-tech radial successors. Many tractors still come from the factory with bias-ply tires, mostly as a cost-savings measure, but also because of their tougher design, especially in the sidewalls, which makes them more puncture resistant than radial tires.

Radial tires are named for their steel cord structure, which unlike bias-ply tires, keep all their cords in once direction, following the circumference of the tire. Radial tires offer better traction, better weight distribution across tread width, a longer tread life and better road manners than their bias-ply predecessors. Their main drawback would be the extra cost associated with their production, which is why they haven’t usurped biased ply tires completely in the agricultural market, and the fact that bias-ply tires still make for a tougher, less puncture prone tire. Radial tires offer many more benefits to road vehicles like your car or pickup truck, but these benefits are seldom realized in a tractor tire that will never see speeds beyond 30 mph.

Load Rating Systems

Modern tires use load index numbers to express their maximum load capacity. These numbers can be found on tires labeled with the metric sizing method. These load rating numbers correspond to the international load index chart, which will decode this number into actual weights.

Radial tires that use the standard sizing method have a star rating system, rewarding the tire with either a one, two or three-star rating which corresponds with their intended inflation pressure; 18 psi, 24 psi and 30 psi respectively. Load ratings will vary depending on the tractor tire sizes and star ratings, but this is usually marked in plain English near the star rating. Along with this information, it will usually state what this load rating is equivalent to in ply ratings.

Ply rating is an old standard born from the days of cotton bias-ply tire construction. Ply ratings used to correspond with the actual number of layers of cotton cloth used, but with the advent of steel cordage in tire construction, the official ply rating system was adopted to reflect the toughness and capacity of the tire, regardless of actual layer counts. Bias-ply tires and old radial designs still use this rating method.

Selecting Proper Tractor Tire Sizes

Even the best tractor for small farms can be rendered useless with the wrong tire installed, so careful consideration should be taken when choosing what size tires to shod your tractor with. Keep in mind that most tractors have a shorter tire up front compared to their rear tire, and this size ratio needs to be preserved if you want your tractor to sit level.

More importantly, most modern farm tractors are 4×4, and changing this size ratio can destroy your gear train. Overall tire height can be changed on a 4×4 tractor, however, you must change front and back tire profiles equally to avoid major damage. Consult your tractor’s manufacturer for guidance if you have questions about different tire size compatibilities, and how they affect the operation of your tractor.

What you Need to Know

In short, the standard method of tire size expression is simple but leaves you hunting for more information like load ratings and style. The metric system is a convoluted expression system that uses; millimeters to show width, a percentage to indicate sidewall height, a wheel size in inches, a load index number you’ll need to cross reference, a speed rating you’ll have to look up, and a style code. Excessive? Maybe, but having all this information makes ordering tires an awful lot easier.

What do you think? Are tractor tire sizes more complicated than they should be?