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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., 7.000-14 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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