TYRE SIZING
Passenger Tyre sizing is typically displayed
as:
P 215 / 65 R 15 89H
|
P
|
The "P" stands for "P-Metric" or
"Passenger". Tyres with higher ply ratings will generally start with "LT" which stands
for "Light Truck". This indicates the tyre is an LT metric and will always have a Load
Range indicated. It is important to note this for vehicles that call for LT metric tyres.
Never substitute a P metric tyre for an LT metric tyre, even if all the other dimensions
are the same.
|
|
215
|
The "215" is the width of a tyre, also
known as the "section width". This is the width of the tyre in millimeters at its widest
point from sidewall to sidewall when mounted on the recommended rim width. The actual
tyre width can vary depending on the rim width it is mounted on.
|
|
65
|
The "65" is known as the Aspect Ratio.
It is calculated by dividing the section height by the section width and multiplying by
100. (In this example, the sidewall will be 65% of 215).
|
|
R
|
The "R" stands for Radial, meaning it
has a radial construction. Radial tyres have ply cords that extend to the beads and are
laid at 90 degrees to the centerline of the tread, the carcass being stabilized by a
circumferential belt. Other possibilities include "B" for belted construction and "D" for
diagonal construction. This means the ply cords extend to the beads and are laid at
alternate angles less than 90 degrees to the centerline of the
tread.
|
|
15
|
The "15" stands for the diameter of the
wheel in inches. This is the exact size that this tyre will fit. There are some older
rims called "TRX" which are metric measurements like 390. You CAN NOT mix TRX rims with
regular tyres or vise-versa.
|
|
89
|
The "89" is the load
index.
|
|
H
|
The "H" is the speed
symbol.
|
SPEED RATINGS
The speed rating of any tyre is a measurement of the
top safe speed the tyre can carry a load under specified conditions. See the chart below for
conversion:
LOAD RATINGS
The load rating for any tyre (load index) indicates the
maximum weight that each tyre is able to support. Below is a quick rating of common Load
Indexes:
OFFSET
The offset of a wheel is the distance from the mounting
surface of the wheel to the true centerline of the rim. A positive offset means the mounting surface of
the wheel is positioned in front of the true centerline of the rim / tyre assembly. This in effect brings
the tyre in to the guard well more. Conversely, a negative offset means the mounting surface of the wheel
is behind the true centerline of the rim / tyre assembly. This will cause the tyre to stick out away from
the vehicle.
BOLT PATTERNS
Each wheel has a different bolt pattern, and some
wheels even have 2 different bolt patterns which allow it to be mounted on a wider range of
vehicles.
Most Bolt Patterns are represented in the following
manner:
4/100
The "4" indicates the number of holes in the wheel for
the bolts to enter and mount the wheel onto the car.
The "100" indicates the diameter of the bolt circle
measured in millimeters or inches. 4 & 6 bolt wheels are measured from the center of one bolt hole to
the center of the bolt hole directly across from it. On a 5 bolt pattern, it is a bit trickier to measure
without special tools. Imagine a circle running through the centers of each bolt hole. You would measure
from the center of one bolt hole to the imaginary circle that lays between the opposite two bolt
holes.
CENTREBORE
This relates to the center hole in the wheel that
centers the wheel on the hub of the car. Since most wheels are mass produced, they have a large center
bore to accommodate several different vehicles. If this is the case, it is recommended that you use a hub
ring. Hub rings are hard plastic or metal ring that fits between the wheel and the vehicle. This centers
the wheel perfectly on the hub ensuring that there is no run out when the wheel is installed on to the
vehicle. Without hub rings it is possible to get vibrations even if the wheel / tyre assembly is
perfectly balanced.
|