HOW TO READ TIRE SIZE DESCRIPTIONS
This is the third of four articles
on automobile tires by John
Thomson. The next article will
discuss a tire’s capability markings.
Tire manufacturers have a number of labeling
conventions. In this article we will discuss the manufacturer labeling
conventions for tire size. All tires sold in the United States must meet the
size standards for bead shape, width, diameter and other parameters that are
agreed upon by the Tire and Rim Association and the European Tire and Rim
Technical Organization. Those standards are virtually interchangeable.
Several tire size systems are in use today, and
the system applicable to a particular automobile depends on when it was made. As
automobile performance increased and suspensions became more complex, successive
tire sizing systems were called upon to provide ever more information about tire
dimensions. Primary data requirements for proper tire fitment are height, width
and load carrying capacity. Over the years various methods were used to describe
tire sizes for automobiles manufactured in particular periods. The tire sizing
conventions for each period continue to be used to describe tires for
automobiles produced during that particular period. Replacement tires in those
original tire sizes are still made today by companies such as Firestone and
Michelin and are readily available from specialized tire suppliers.
The generally accepted passenger tire sizing
conventions are: numeric, the oldest standardized tire
sizing system for passenger car tires, based on a tire’s aspect ratio; alpha-numeric,
a system established in 1968, which added a tire’s load carrying capacity in
addition to its aspect ratio; p-metric, the United States
version of the metric sizing system established in 1976; and metric,
the European metric tire sizing system similar to the American p-metric system.
Before we go any further, we have to understand
what is referred to as "aspect ratio." Put simply, the term refers to
the relationship between an object’s width and its height, but contemplates
more than that when it comes to tires.
A tire’s aspect ratio is the relationship
between its width and its height. A tire’s height, sometimes called its
section height, is measured from the inside of the tire to the outer tread
surface when it is fully inflated. Its width, sometimes referred to as the
section width or cross-width, is the distance between the outsides of the
sidewalls when the tire is fully inflated. Note that a tire’s aspect ratio
percentage is multiplied by 100 in order to express the aspect ratio simply as
an integer, or whole number, on the tire’s sidewall.
The importance of aspect ratio lies in the
connection between a tire’s aspect ratio and its performance. Here are three
tires with different aspect ratios: 65, 60 and 55. The arrows represent lateral
force exerted on the tire, force that would act on a tire when cornering or
rounding a curve.
Note that the lower aspect tire responds more
effectively to lateral force, with less sidewall distortion than a tire with a
higher aspect ratio. The lower aspect ratio also affects steering stability.
Generally, the shorter the sidewall, or the lower the aspect ratio, the less
time it takes to transmit steering input from the road wheel to the tread. The
result is quicker steering response. Aspect ratio also affects the tread contact
patch, or footprint, which is the area of the tire’s tread that is in actual
contact with the ground. As a rule, a low profile tire produces a wider tread
contact patch, creating a stiffer footprint that reduces distortion and provides
improved cornering traction.
On the other hand, a low profile tire usually has
a stiffer ride than a standard tire with an aspect ratio of 75 or more. So the
general rule is that the higher the aspect ratio, the more comfortable the ride,
and the lower the aspect ratio, the better the handling. It is up to the
individual driver to decide which tire characteristic is desired. Now let’s
look at the various sizing conventions used over the years.
The numeric method is the oldest
tire sizing system for automobile tires. This system only provides the
cross-section width of the tire and the rim diameter in inches. Two versions of
numeric tire sizes were used as original equipment on vehicles between 1949 and
1970. The early numeric tires had the equivalent of a 90-series aspect ratio,
while later tires offered a "lower" profile equivalent to a 80-series.
These tires typically featured tread widths ranging from 3.5 inches for the
smallest 13 inch rim diameter tires to about 5.5 inches for the largest 15 inch
rim diameter tires. If the section width ended in zero (e.g., 7.00-14 or
7.50-14) the tire had an aspect ratio of about 92. For section widths not ending
in zero (e.g., 8.25-15) the tire was considered "low profile" with an
aspect ratio of about 82. For example, a tire labeled 7.00-14 has a section
width of 7 inches, a rim diameter of 14 inches, and an aspect ratio of 92. The
low profile equivalent size tire with an aspect ratio of 82 would be 7.35-14.
In 1968, the alpha-numeric sizing
system was introduced. This is a load-based system where tires are designated by
their load-carrying capacity and aspect ratio. The first letter is the load and
size relationship, with letters ranging from A to N. The closer to A, the
smaller the size and, of course, the lower the load-carrying capacity of a tire.
The alphabetical load rating introduced by the alpha numeric system was a
precursor to the numerical load index that appeared later in the metric sizing
system. The "R" indicates that a tire is of radial construction, then
comes the aspect ratio, followed by the rim diameter.
The p-metric size system is the
U.S. version of the metric sizing system that was established in 1976. P-metric
passenger car tire sizes begin with a "P," which stands for
"Passenger," and means that the tire is engineered to standards set by
the TRA. If there is no "P" the tire is engineered to ETRTO standards
and is a metric tire. Then comes the section width in millimeters, the aspect
ratio, the radial designation, and the rim diameter. The p-metric system is
widely used by U.S. domestic tire manufacturers and is essentially a conversion
of the numeric system in which section widths are expressed in millimeters
instead of inches. TRA standards also generally allow for a higher maximum tire
pressure for a given p-metric size so as to give the tire less rolling
The metric sizing system is a
European tire sizing system similar to p-metric. The absence of the
"P" designator means a tire is engineered according to ETRTO
standards. So we start with the section width in millimeters, the aspect ratio,
the rim diameter, the load index, and the speed symbol. Like the p-metric, the
metric sizing system is essentially a conversion of the numeric system where
section widths are expressed in millimeters instead of inches. The load index is
a numerical code ranging from 70 to 131 and is associated with the maximum load
a tire is rated to carry. A higher number indicates higher load capacity. We
will discuss the speed symbol in a later article.
While today's p-metric passenger tire sizes have
existed since the early 1980s, the growing number of classic cars has kept
yesterday's numeric and alpha numeric tire sizes from disappearing. However,
some method of establishing equivalence between old and new tire size systems is
important to owners of older cars. Though tire conversion charts are available
on the Internet and from most tire dealers, I will outline the basic principles
of converting the earlier numeric and alpha numeric tire sizes to today’s
To convert numeric sizes to today's p-metric
sizes, it is important to remember that early cars were not only equipped with
narrow tires, they were equipped with narrow wheels as well. Note that in
discussing tire size conversions, we usually refer to aspect ratios as
particular "series" of tires; that is, a tire with an aspect ratio of
80 is referred to as an "80-series" tire. In most cases, numeric tires
should be replaced with today's 80- or 75-series size tires. This is especially
important if the original wheels are to be used. Today's lower profile sizes
will usually result in too wide a tire with too much gap between the wheelwell
opening and the top of the tire.
To convert alpha numeric tires to p-metric sized
tires, it is important to identify the original tire's aspect ratio. The
78-series alpha numeric tires should be replaced with today's 80-, or 75 series
tires. If the vehicle was equipped with the low profile 70-, 60- or 50-series
sizes, when using a conversion chart the p-metric substitution should be
selected from the p-metric size column that offers the equivalent aspect ratio
as the existing tire. Remember that while tire sizes are generally
interchangeable by using a conversion chart, it is recommended that you use the
type of tire for which the car was originally designed, unless the car is
modified to use radial tires by replacing the wheels and adjusting the
suspension. Also, you should never mix radial and bias tires on the same car.