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
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
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.
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
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.
Millimetric Sizing System:
The Millimetric sizing system is
similar to the Metric system except that the rim diameter
is also represented in millimeters.