American Tire Sizes

[Zebedeo Konig]

Theseclassifications originate from the time of tire brakes. In those days, the exact outer diameter of the tire was defined by the brake. Depending on tire width, various standards for the inner diameter applied.

Inch dimensions are widely used in both MTB sport and English language countries. Therefore, we will continue to use these specifications for all tires. But only in decimal form, e.g. 26 x 2.25. Experience tells us that nowadays very few users are familiar with the classical fractional inch dimensions, such as 28 x 1 5/8 x 1 3/8.

When the 29 inch MTB tire size was introduced a few years ago, it had the same inner diameter of 622 mm, known as 28 inch in Europe. The latest tire size is 27.5 inches. This tire size is favored for MTBs which are too small for the very big 29 inch wheels, in order to benefit from the advantages of bigger diameters. This is the case, for example, with bicycles having a very long suspension travel or very small frame sizes. 27.5 inch tires have an inner diameter of 584 mm and are identical with the old French size marking 650B.

French size markings (e.g. 700 x 35C) give the approximate tire outer diameter (700 mm) and width (35 mm). The letter at the end indicates the inner diameter of the tire. In this case, C stands for 622 mm. French size markings are not used for all tire sizes, so, for example, it is not used for MTB sizes.

The new sizes are well established on the market and this is for good reason. The advantages of the large wheels outweigh the disadvantages. 29 inch wheels weigh a little more, are less maneuverable and possibly less stiff, but they roll clearly faster on rough surfaces especially in the terrain. The contact surface is bigger which makes for a significantly better tire grip.

Your vehicle manufacturer manual will provide guidelines for proper maintenance. You can count on us to help you take care of your tires and your vehicle, and provide expert auto services when you need them.

On the road, the safety of every driver and passenger depends greatly on the quality of the tires that carry them. Despite that fact, tires are likely something the average person does not give much thought to until they need to be repaired or replaced. For every vehicle owner, it is worth taking a little time to learn about tires. While our Tires 101 section will not make you a tire expert, it will provide you with a basic tire overview and get you a little better acquainted with your tires.

Tires are comprised of many different elements, which are assembled in a TBM, or Tire Building Machine. After assembly, tires are cured and finished to precise specifications. The basic components of a tire include:

The kind of tire and the use intended are indicated by the first letter in the code. Letter designations include P primarily for passenger vehicles, T for temporary spare, LT for light truck metric, C for commercial, and ST for special trailer service. Euro-metric tires do not include this first letter descriptor.

Before the slash mark, and following the first alpha character, the section width of the tire is listed in millimeters. This is the widest point from sidewall-to-sidewall, so a larger number indicates a wider tire. In this case, the tire has a width of 205 millimeters.

Following the slash mark is the aspect ratio, which is described as a percentage and gives the height of the tire from the bead to the top of the tread. Here the number is 55, which means the tire height is 55% of the section width of 205 millimeters. The height of this tire would be 133.25 millimeters. A lower number indicates a lower tire profile

The construction of the tire is indicated by the letter following the aspect ratio. The most common designation is R, which stands for radial construction. Other less common construction types for modern passenger cars may include D for bias ply construction and B for belted tires.

The number following the construction code is the size of the wheel that the tire will fit in inches. The example tire would be designed to fit a 16-inch wheel. Tire sizes on modern vehicles typically start at 13-inches and go up to 18-inches, but custom package wheels can be 22-inches or even larger.

Sufficient tire tread depth is an essential factor in vehicle handling and traction. In fact, tire tread depth is so important, US law requires tires to have plainly visible tread wear indicator bars. However, these bars can be difficult to see until the tread is well worn. The easiest way to measure tire tread depth, and be assured of the safety of your tires, is with the tried and true penny test.

In choosing tires, it is important to consider factors such a quality brand and solid value, but it is also essential to select the right tires for your type of vehicle. It is also imperative that driving conditions be a major consideration in making a tire selection. Several types of tires are available to suit every kind of vehicle and all driving conditions.

All season tires are a widely used and most popular variety of tire, carrying S and T speed rating. Sedans and minivans commonly have all season tires as standard. All season tires are developed to handle most conditions from dry pavement to wet weather and moderate snow. Characteristics of all season tires include a comfortable and quiet ride, reliable handling, as well as long tread life. All season tires are a good choice for most drivers, except where winters are severe. For year-round traction in most climates, all season tires are an excellent choice.

A tread with gripping edges for better handling on snow and ice distinguishes winter tires from all season tires. Often winter tires are developed with a softer rubber compound that remains flexible in temperatures of extreme cold. Winter tires are not the best performers for driving on cleared roads because they tend to not grip as well as all season tires and tend to wear more quickly. Winter tires should be used during extreme cold weather driving conditions. Winter tires should be used in sets of four for balanced handling and optimal grip for braking in snow and icy conditions.

Drivers looking for the all-around best performing tires in mild climates and seasons will find summer tires offer a performance level above all season tires. Summer tires, as the name implies, are not at all suited for driving in snow and ice, but on dry and wet roads in mild temperatures they offer solid handling. This is because the softer compounds used in summer tires, unlike those used in winter tires, become harder in colder temperatures. While the performance is enhanced with summer tires, they tend to have shorter life span and more rapid tread wear.

Run-flat tires are a self-supporting type of tire that can be used on light trucks and passenger cars. Developed with much thicker sidewalls than conventional tires, run-flat tires are able to support the weight of the vehicle even when entirely deflated. Run-flat tires are capable of operating without air pressure for 50-200 miles at 50-55 MPH (depending on the tire) while maintaining virtually the same level of ride comfort and handling. Since any loss of air pressure would not be detected by the driver, vehicles using run-flat tires must be equipped with a tire pressure monitoring system. If run-flat tires are driven too long without the proper tire pressure, they may become irreparably damaged.

When pneumatic tires hit the mainstream in the 1900's, tires were inflated to 65 psi and higher, in order to hold the tire to the rim properly. We now refer to these tires, such as the Goodrich Silvertownd Cord (pictured), Firestone Non Skid and others, as "High Pressure" tires. Whether it's a clincher rim, lock ring style or other type of Brass Era rim, these high pressure tires feature an early sizing style.

A common size for a high pressure tire is 30 x 3. The "30" in the sizing refers to the tire's overall diameter and the "3" is an approximation of the tire's section width--simple enough, right? The only thing left out of the size is the rim size. To determine the rim size on high pressure tires, we double the section width number (3 + 3 = 6) and then subtract it from the diameter (30 - 6 = 24 inches).

Another example is 34 x 4 1/2 (these are always written out in fraction form, not decimals). In this example, our quick math tells us that it fits a 25-inch rim. High pressure tires are common on vehicles built before 1927. They would eventually be phased out by Balloon tires and drop center rims, which used less air pressure.

When this new style tire and wheel hit the market, it had a new sizing structure that we now refer to as numerical sizing. It leads off with the tire's section width, in inches and then features its rim diameter (ex. 4.50-21, sometimes referred to as 4.50x21). In the case of a 1930 or 1931 Ford Model A, the original tire size was 4.75-19. Through the years, popular tire sizes were combined, resulting in the current availability of 475/500-19 tires. This combination of similar sizes mostly applies to tires produced in the 1920's and 1930's.

Many of the early Numeric sized tires feature nearly a 100 percent aspect ratio, meaning that the sidewall section height is nearly equal to the section width of the tire. In those cases, you can get a rough estimate of the tire's overall diameter by doubling the section width and adding the rim diameter. 5.25-21 tells us that 5.25 + 5.25 + 21 = 31.50, which is the exact dimension of the tire's overall diameter. This math doesn't always add up to the exact number, but it gives you a close estimate.

Going in the 1930's the most common tire size was 6.00-16. Again the 6.00 refers to the section width of the tire, resulting in a tread width of about 4-1/2 inches. After World War II, the automotive world began changing, with futuristic designs, creature comforts and more powerful engines.

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