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READING YOUR TIRE
TIRE SIZING SYSTEM
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Numeric Sizing System
This first system developed for tire sizing was used until the
late 1960s, but provided only the cross section width of the tire
and the rim diameter in inches. If the section width ended in zero
(e.g., 7.00-14 or 7.50-14), the tire had a common 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.
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Alpha-Numeric Sizing System
In 1968, a new concept was introduced worldwide. The Alpha-Numeric
sizing system 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 lower the letter, the smaller the size and, of course, the lower
the load-carrying capacity of the tire.
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P-Metric Sizing System
To accommodate the smaller tires used on compact cars, the P-Metric
(Passenger Metric) system was created in 1976. The maximum inflation
pressures of P-Metric tires were raised for lower rolling
resistance. The P-Metric system is widely used by domestic tire
manufacturers.
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Metric Sizing System
Because Europe primarily uses the metric system of measurement,
the metric sizing system was developed. It is essentially a
conversion of the Numeric system. Section widths are notated in
millimeters instead of inches. Originally, tires not identified
with an aspect ratio were assumed to be 82-series. When 60- and
70 series tires appeared, the aspect ratio was added to the
nomenclature, similar to the P-Metric system.
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ISO Metric Sizing System
The International Standards Organization (ISO) Metric system
combines the Metric system with a service description. The service
description provides the load index along with the speed rating
symbol.
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Millimetric Sizing System
The Millimetric sizing system is similar to the Metric system
except that the rim diameter is also represented in
millimeters.
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Light Truck Numeric System
Similar to the Numeric system for cars, it lists the section
width in inches, construction type, rim diameter in inches, plus
the light truck designation.
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Light Truck Metric Sizing System
Similar to the P-Metric system, except the P is replaced with the
LT light truck designation. Also, LT-Metric and P-Metric tires
differ in construction.
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Light Truck High Flotation System
The same as the Light Truck Numeric system with tire diameter
added to the front.
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High Flotation vs. LT-Metric vs. LT-Numeric
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