Explanation of Tire Data Fields
company manufacturing this tire.
carry a variety of tires for various applications, depending on the needs
of the driver.
= Extra Narrow White Sidewall
Extra Narrow Segments
of tire without inflation or rim.
Size & Service Description (e.g. P185/70R13 89T):
Tires with a P are using a P-Metric designation, which is the American
market sizing standard started in 1976. All P-Metric tires are engineered
to standards of T&RA (Tire and Rim Association). The "P" stands for
"Passenger" vehicle. If there is no "P", the tire is engineered to ETRTO
(European Tire and Rim Technical Organization) standards and is considered
a metric tire. Standards by ETRTO and T&RA have evolved together, and
are virtually interchangable.
The first number on the tire size is the cross-section width. This value
is the width of the tire from side-wall to side-wall as measured in millimeters.
This measurement is usually obtained from the bead of the tire (where the
tire mounts to the rim.)
The two digit number just to the right of the slash (/) is what is referred
to as the "aspect ratio". Not a 'specific length', this value describes
the distance between the bead of the tire to the tread as compared to the
cross-section width. This is viewed as a percentage value. (e.g. P185/70R13
has an aspect ratio of 70% of 185mm....or 129.5mm.). Lower apect
ratios confer a stiffer ride and increased control than a similar sized
tire with a larger aspect ratio. But larger aspect ratios tend to be more
absorbant and add to a smooth road ride.
This letter indicates the type of ply construction in the tire's casing
or carcass. "R" means radial. "D" means diagonal, referring to bias tires.
"B" means belted for belted-bias tires. Never mix radial tires with any
other constru ction on a car.
Rim Diameter. It is the diameter of the bead seat in the rim. Most
tires are built to inch standards for rim diameters. However, some tires
are built to millimetric rim dimensions. Always match the tire's rim diameter
to the wheel rim diameter.
Load capacity. Refer to the load index chart of a tire guide to find the
maximum load carrying capacity of the tire.
Speed Rating. This designation should be compared to the Speed Rating
Chart to view the recommended maximum speed for this tire.
is the suggested rim width range that the specific tire should be mounted
to. The number in parenthesis is the rim used to obtain other values in
this chart, such as over-all width.
diameter of an unloaded, inflated tire measured from the crown on one side
to the crown on the opposite side. The free radius equals one-half the
overall diameter. Sometimes called the outside diameter.
is the widest measurement of the tire from side-wall to side-wall. The
width of tire buldge at full inflation.
is the width of tread, side to side, in inches that actually contacts the
road suraface when tire has full inflation.
distance from the center of the axle to the ground under the specified
load at full inflation.
per mile. The number of full rotations the wheel completes when a linear
mile is travelled.
depth of grooves and channels on the wheel's contact surface. Deeper tread
depth permits water to drain away from the footprint, and may increase
in traction on snow or soft surfaces.
milage up to which the tires may be driven on and still covered by the
manufacturer's warranty against defects.
is the Uniform Tire Quality Grading system. It is a series of a number
and two letters such as 320AB that represent the "3 T's" of tire grading:
Treadwear, Traction and Temperature ratings. The treadwear grade is based
on a standardized government test with a base index of 100. A tire
with a 320 treadwear rating would wear 3.2 times better than the government's
"base" tire. The traction letter (A, B or C) represent the tire's ability
to stop on wet pavement. "A" tires have the best wet pavement
traction in straight-ahead braking tests. The temperature letter (AA, A,
B or C) represent the tire's ability to dissipate heat and resist heat
under controlled test. "AA" tires have the best heat resistance.
for these wheels are for vehicles capable of over 150mph/240 kph but NOT
exceeding 168mph/270kph or 185mph/300kph respectively.
to Tires For Spitfires Page