Tire Sizing

Sizing Systems

The first 3 numbers of the size measured the width of the tire casing at its widest point. For example, the 6.00 of the size 6.00X13 is 6 inches wide. This called the section width.

NUMERlC sizing is still used today on such smaller tires as camper trailer tires, etc. An example would be 5.70X8.

The 13 of a 6.00X13 is the diameter of the wheel, in inches, that the tire is to be mounted on. For example:

So, with numeric sizing we know the Section Width of a tire as well as the diameter of the wheel it is to be mounted on.

For the vehicles these Bias Ply, Numeric Sized tires were going on, this was enough.

Alpha-Numeric Sizing

Some size examples are:
A78X13 E78X14 G78X15
B78X13 F78X14 H78X15
C78X14 G78X14 L78X15

Notice that letters of the alphabet have been included in the sizing; thus the term, Alpha-Numeric.

As American car manufacturers increased the size and weight of automobiles and the pickup truck became more popular, it became necessary to know the load range or load carrying capacity of a tire. The Department Of Transportation (D.O.T.) set up a system using letters of the alphabet, starting with the letters A, B, C, D, E, etc. to rate the load range of a tire.

There are 2 basic ways to increase the load carrying capacity of a tire:

1. By increasing the air pressure.
2. By increasing the size of the air chamber and going to a larger tire.

You'll notice some number changes with Alpha-Numeric sizing vs. Numeric sizing: the addition of the 78 or 70. The D.O.T began to include what is called an ASPECT RATIO.

What is Aspect Ratio?

The operative word of the two-word definition Aspect Ratio is RATIO. Whenever there is a ratio there is a percentage of something; in other words ? VS ?. When relating to the aspect ratio of a tire it is HEIGHT vs. WIDTH. The sidewall height of a tire is measured as a percentage of the width of the tire at the widest point of the casing.

Example of ASPECT RATIO, Size D78-14

This tire's sidewall height is 78% of its section width, which we do not know. Alpha numeric size systems did not give us section width measurements. If this were a metric 215/60R15, the sidewall height would be 60% of 215 mm. The 215 mm. is the section width of that tire at the widest point in the casing. Be patient, we will discuss metric size systems soon.

The height to width relationship determines the shape of the tire on the rim, and, more importantly, determines the performance characteristics of the tire. If the sidewall height of a tire is reduced slightly, the sidewall stiffness is increased greatly.

Higher Aspect Ratios deliver greater deflection under load and a softer ride. Lower aspect ratios deliver a wider footprint, quicker response, less slip angle, lower flex rate, less deflection and a harsher ride.

Metric Sizing

You will notice there is little similarity here in some of the numbers to numeric or alpha-numeric sizing except for the last 2 digits; 13,14 or 15 which is still the diameter of the wheel, in inches, that the tire is to be mounted on. However, where the numeric system used a 6.00 or 7.00 to denote the cross-section of a tire in inches, the Metric system uses the numbers 165 from the 165SR13 to denote the measurement of the cross-section in millimeters.

For Example:

You'll also notice that some of the example Metric sizes have two extra numbers like 70 or 60 in the 175/70SR13 and 215/60HR15. These are the aspect ratios like 78 was in A78-13.

Why do some sizes have aspect ratios and others do not?

Radialization and technology increased the tire manufacturer's ability to improve the performance of the tire dramatically by increasing the section width and reducing the sidewall height of the tire. This change of the aspect ratio needed to be reflected when stating the size. An example of section width increase in size on the same wheel diameter would be 155"82"SR13 or 155SR13 to 175/70SR13 or 205/60R13. All 3 sizes essentially have the same sidewall height but notice the section widths increased from 155 mm to 175 mm and 205 mm as the aspect ratio respectively changed from "82" to 70 to 60. The overall tire diameters of these 3 sizes should be within 3% of each other.

For Example:

Why does a tire get taller as the first three numbers get larger?

Remember, the "75" is an Aspect Ratio, a percentage measurement, not a specific measurement like 205 mm at the cross section or 15" wheel diameter.

So, 75% of 225 mm is going to be taller than 75% of 215 mm, and 75% of 215 mm is going to be taller than 75% of 205 mm, and so forth.

For Example:

What is a P-Metric?

Radial tires had, for the most part, replaced bias-ply tires. The United States was hit by its first major fuel crunch in the mid-70's. Our American appetites for driving import cars increased dramatically. Both Japanese and European cars were much more fuel efficient since they had gone through a fuel economy crisis some 20 years earlier and the European system of Speed-Rated tires was invading the North American continent at a rapid pace, all in Metric sizes.

Metric tire sizing was much more efficient than Numeric or Alpha-Numeric. Metric tire sizing showed the cross-sectional width and Speed-Rating of a tire as well as the Aspect Ratio.

For Example:
ALPEA-NUMERIC: DR70-14
METRIC: P195/70SR14

The D.O.T. decided to add the "P" to P-Metric to denote that it was designed for passenger car use for load carrying capacity limits. Remember, we are already using letters of the alphabet in our older alpha-numeric system to denote the load range of a tire.

Why would we put a passenger car tire on a Light Truck application?

Light truck rated tires of load range C or greater tend to be rougher riding and noisier than their P-Metric counterparts and more expensive since the LTR size equivalent to the P-Metric has heavier body ply construction and the tread lugs are generally more aggressive. Light truck manufacturers are delivering the vehicles, new, from the factory with P-Metric sizes already on. For instance, where do we see these P-Metric sizes with the most frequency?

        P205/75R15      LT205/75R15
        P215/75R15      LT215/75R 15
        P225/75R15      LT225/75R15
        P235175R15      LT235/75R15

Even though some of the 5 to 15 year older domestic passenger cars still use the P-Metric sizes listed above you'll see them with greater frequency today on new light truck type vehicles.

What are the extra numbers and letters at the end of the P-Metric size?

For Example:
P215/60R15 92H

In this example the 92H is called the SERVICE DESCRIPTION. The Service Description consists of two parts, the LOAD INDEX and the SPEED RATING. You'll notice the Speed Rating "H" has been moved from the old Metric size system of:
215/60HR15 to P215/60R15 92H

The SPEED RATING is a letter which designates the rating achieved on indoor wheel testing.

The LOAD INDEX is a number ranging from 0 to 279 and covers load capacities from the smallest motorcycle tires to those for the largest earth mover tires. In passenger car tires, the load index typically ranges from 75 to 100.

As in aspect ratio where the operative word was ratio, in load index the operative word here is index. An index is a point of reference or a chart to refer to find information. All tire guides used by retail tire stores have them and they have load index charts in them. By referring to the load index chart in the Tire Guide you can determine the maximum load carrying capacity of a tire size from using the load index number at the end of the P-Metric size.

For Example:

Load Index   Max Load (lbs.)
    91           1356
    92           1389
    93           1433

General Sidewall Construction

Sizing D.O.T. Summary

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