CLASSIC CAR TIRES 101:
ALL YOU EVER WANTED TO KNOW ABOUT CAR TIRES

BIAS PLY AND BIAS BELTED CONSTRUCTION

The construction of a tire goes much deeper than what you see on the outside surface. The body of a tire is referred to as a "casing" in the industry, and is composed of multiple layers, which are called plies or belts. Belts under the tread give the tire strength, prevent the rubber tread from undulating, and provide resistance to puncture. The materials used for bias ply tire construction over the years have included rayon and nylon, with rayon more commonly provided as original equipment and nylon being offered as an extra cost option. In the late sixties, fiberglass belted tires were a popular option on many performance cars, as the fiberglass made the tire stronger and improved the useful life of the tire.

As the tire casing is being constructed, the various layers (plies) are laid on top of each other in pairs. The innermost plies are normally placed at a 45 degree angle, or bias, which means the plies are placed diagonally to each other, forming an "X" as they run from one bead of the tire to the other (inside edge to outside edge). The outermost plies run lengthwise around the circumference of the tire. The purpose of this is to provide additional strength to the tire. Most tires made for passenger cars during the late 1940s-early 1960s were built with 4-ply construction, which meant they had 2 layers, or 2 pairs of plies, which made them 4-ply tires.

This type of tire construction makes the sidewall area of the casing very strong, which allows the tire to support more weight. This was very important at the time as cars were beginning to offer additional equipment such as power steering, air conditioning, and lots of other accessories that greatly increased their overall weight. The tread area of bias ply tires is very firm, and this design ensures good contact of the tread with the road. In fact, the design of this type tire can cause a very firm, even harsh ride. This is one of the reasons many cars during this era were so softly sprung, in an attempt to provide a comfortable ride for passengers.

In addition to the ride characteristics, this type of construction doesn't flex very much, which creates rolling resistance and additional heat as the tread area of the tire contacts the road. This heat can build up to a point where it actually causes the tire to wear faster than normal, and is compounded by an under inflated condition, as well as poor vehicle alignment.

Bias ply tires are also prone to developing flat spots, especially if the vehicle sits for extended periods of time. Sometimes these flat spots are permanent, and cause poor ride and handling. However, it is possible for the tire casing to return to its normal shape as heat builds up when the vehicle is driven.

Vehicle tires and wheels at this time were narrow compared to today's standards, which creates confusion when attempting to install new tires on a vintage car today. Today's tires have a lower profile, with a wider tread area which can cause appearance problems involving ride height with older cars. It is also important to make sure proper clearances are maintained for steering, as some of the built-in factory tolerance computations were quite close. For additional information on specific tire sizes for your classic car, please refer to our Tire Size Conversion Guide (planned future site addition).

RADIAL PLY CONSTRUCTION

Radial ply tires are constructed in many ways much the same as bias ply tires, with a couple of notable exceptions. The innermost plies on a radial tire are not angled as much as bias ply tires. Radial plies normally have only a 2 degree or less angle to the plies, which makes them concurrent to one another, running sideways across the tire from bead to bead. The belts in a radial ply tire are normally steel.

Radial ply tires are more flexible than bias ply tires, which allows the tread and sidewall of the tire to conform to the contour of the road better, especially during cornering. The result is reduced rolling resistance, improved handling and traction under all conditions, and a reduction in heat build up, when properly inflated. The reduced rolling resistance of radial ply tires also allows for a notable increase in gas mileage, often well over 10%.

Radial ply tires are less likely to fail due to blowout, and are more resistant to puncture when kept inflated at proper pressures. The greater flexibility also allows for a lower aspect ratio, which is covered below.

NUMERIC SIZING

American tire manufacturers used to use numerals to denote the various tire sizes. A few examples are:

6.00x12, 7.50x14, and 7.10x15

The first three numbers indicate the size of the tire, by measuring the casing at its widest point. Also referred to as section width, the first size listed above (in bold), as an example would be 6 inches wide. (6.00 = 6 inches)

The last numbers indicate the diameter in inches of the wheel that the tire is designed to be mounted on. Using the same example above, the wheel would be 12 inches in diameter. So, the tire has a section width of 6 inches by a diameter of 12 inches, or 6.00x12.

ALPHA-NUMERIC SIZING

With cars becoming heavier and more demands being placed on them by drivers, the numeric sizing no longer provided enough information to ensure the tire would perform properly under the harsher conditions it would be exposed to. Since weight plays such a critical role in tire safety, new alpha-numeric sizing was introduced to provide this information in the form of a load range rating. A few examples:

A78x14, G78x14, and L78x15

Using the example in bold above, the letter at the beginning denotes the load range rating or load carrying capacity of a tire. This designation was created by the Department of Transportation (D.O.T.) by using letters of the alphabet to indicate the load range of a tire. The letters "A" through "L" are used, skipping the letter "I" since it could possibly be confused for the numeral "1" (one).

The load carrying capacity of a tire is a mathematical formula which considers the physical size of the air chamber as well as how much air pressure is in that chamber.

To increase the load carrying capacity of a tire, you have to either increase the air pressure of the tire or increase the size of the air chamber in the tire by going to a larger sized tire. The letters of the alphabet relate to the load range/air chamber size with "A" being the smallest, "B" the next larger, etc. For example:

A78x14 would be a smaller load range than B78x13

With this change, the D.O.T. started including the Aspect Ratio as well, which is why the numbers immediately after the load range letter are different.

ASPECT RATIO

The Aspect Ratio of a tire is the relationship of its height to its width when mounted and properly inflated on a wheel rim of the correct size for the tire. This is expressed as a percentage of section height to section width in a two-digit number. For example:

80, 70, 60

Also referred to as the tire's series, the combination of a tire's section height and section width is expressed as a percentage, or .70, for example. A tire with this configuration would be a 70 Series tire. To determine this you would use this formula:

Tire Section Height vs. Tire Section Width = % of Tire Section Width at the widest point of the casing is the Series number of the tire.

For example, if you had a tire size of G78x15, the sidewall height would be 78% of its section width. The relationship of section height to section width determines the shape of the tire on the rim, and therefore, its characteristics under various load conditions. This is why radial tires appear to be under inflated at times, even though pressure in the air chamber is correct. A sidewall height with a smaller percentage to width ratio would exhibit more sidewall stiffness, a harsher ride, a wider tread area on the road, and quicker response to driver input.

Raising the Aspect Ratio allows the tire more deflection under load and a softer ride, while at the same time providing a smaller tread contact with the road and slower response times. Confused? A 50 Series tire would have approximately 50% as much sidewall height as it would overall width, which would give it a harsher ride and quicker response than a 60 Series or a 70 Series tire would.

EURO-METRIC SIZING

At about the same time domestic tire manufacturers and the D.O.T. were changing from Numeric Sizing to Alpha-Numeric Sizing, the European tire manufacturers also changed to Metric Sizing, which we refer to as Euro-Metric to prevent confusion. This change occurred due to the change from bias ply tire construction to radial ply tire construction in Europe. Examples of Euro-Metric Sizing would be:

185R13, 175R15, and 215R15

Using the example in bold above, the first three numbers express the tire's section width in millimeters, just the same as the old Numeric Sizing expressed this measurement in inches. The "R" indicates the tire is of radial ply construction, and the last two digits express the wheel diameter the tire is designed to fit.

It's important to note that some Euro-Metric tires have Aspect Ratios listed in the sizing, while others don't. This is due to the fact that the Euro-Metric system was originally set up with an Aspect Ratio of "82" and it wasn't included in the sizing at that time since all tires had this as the default.

Over the years, technology allowed improvements in tire performance by increasing the section width of the tire, and reducing the height of the sidewall of the tire. Since there could now be variations in the Aspect Ratio, this number would need to be included when stating the size of the tire. An example of a Euro-Metric tire size would be:

215/75R15

This example shows a section width of 215 millimeters, an Aspect Ratio of 75, radial ply construction, and a wheel diameter of 15 inches.

P-METRIC SIZING

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This is an easy one. P-Metric Sizing is virtually the same as Euro-Metric Tire Sizing, except the D.O.T. added the "P" to indicate that the tire was designed for passenger car use for load carrying capacity limits.

During the late 1970's and early 1980's, America was hit by a fuel crisis that changed the way Americans looked at their large cars. Europeans had been dealing with high fuel prices for some time and had developed smaller, more fuel efficient cars to maximize fuel economy. Radial tires with their lower rolling resistance were a big part of the overall fuel economy. P-Metric Sizing provided more information than previously available, showing not only that the tire was designed for passenger car use, it also indicated the section width, Aspect Ratio, type of construction, and wheel diameter size.

Using the same example above in Euro-Metric Sizing, this is how it would look in P-Metric Sizing:

P215/75R15

The "P" denotes passenger car load carrying capacity, the "215" reflects the section width in millimeters, the "75" is the Series, "R" is radial construction, and "15" is the wheel diameter size.

P-METRIC SIZING FOR HIGH PERFORMANCE TIRES

Performance versions of P-Metric Sized tires will often add two or three additional numbers followed by a letter to the end of the sizing. This indicates the Service Description of the tire. To get this code, two things are considered: the Load Index and the Speed Rating of the tire.

The Load Index is a number between 0 and 279, and covers all load capacities for all tires. From the smallest motorcycle tire to the largest heavy duty equipment tires, they all receive ratings within this numerical range. Passenger cars tires normally have load indexes rated from 75 to 100. We have provided a Load Index Chart below listing load indexes from 74-150.

The Speed Rating is a letter (or symbol) which designates the rating the tire received in indoor wheel testing, normally using the European ECE 30 Indoor Wheel Testing model as the standard.

Load Index Chart

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