You have to know what the codes mean on your tires first.
An example: Metric P185/80HR13
"P" = Passenger LT=Light Truck
"185" = 185 cross sectional width in millimetres at the widest point of the tire bulge
(Not the tread width at the road surface)
To find Height (0.80 * 155 = 124 mm)
"80" = Aspect Ratio (Height / cross sectional width)*100
An aspect ratio of 70 means the height of the tire is 70% as high as it is wide.
The aspect ratio is related to the Load Carrying capability which is in the next example.
The Load Carrying capability is related to the amount of air in the tire too.
"H" = Speed Rated Code, Tells you the typical top speed rating of the tire.
ZR= >300 km/h
"R" = Type of Construction (R is for Radial)
B=Belted For conventional tires, I don't think there is a letter.
"13" = The diameter for the rim size (in inches this time).
There is also a new code being used. eg. 205/60R15 82S
205 = 205 sectional width in millimetres
60 = Aspect Ratio (60% of 205 = 123 mm high sidewall)
R = Type of Construction
15 = Diameter of rim in inches
82 = Load Index (The list of load indexes is further on)
S = Speed Rating Index of 112 MPH certified by the manufacturer
Each Company also puts on it's own codes such as 4S or +4 for all season or
M+S for Mud and Snow. Easy to get confused here.
THE DEPARTMENT OF TRANSPORT (DOT) CODE:
DOT MA L9 ABC 0409
DOT is just that it meets DOT (Dept. of Transportation) specifications.
MA is Manufacturer and plant code number.
L9 is tire size and code number.
ABC is a group of up to four symbols optional with manufacturer.
0409 is the date of manufacture.
The last three numbers represent the week and year of manufacture.
You should look for a "fresh" tire since time, moisture, heat, pollution and
ultra violet light can effect your tires. Also make sure your tires have not been
stored around gas or oil.
DOT 449 44= forty fourth week of the year, 9=1999
2006 - New U.S. DOT Tire Identification Number
This begins with the letters DOT and indicates that the tire meets all federal standards.
The next two numbers or letters are the plant code where it was manufactured, and the
last four numbers represent the week and year the tire was built. For example,
the numbers 2210 means the 22nd week of 2010. The other numbers are marketing
codes used at the manufacturer’s discretion.
This is based on 100. When you see Treadwear 440, this means the tire will
wear 4.4 times normal. 260 means 2.6 times normal. Basically it is just a
relative measure so you can compare tire wear.
The treadwear grade is a comparative rating based on the wear rate of the tire
when tested under carefully controlled conditions. For example, a tire graded 200
should have its useful tread last twice as long as a tire graded 100. However,
real world tire tread life, in miles, depends on the actual conditions of their use.
Tire life is affected by variations in driving habits, service practices...such as:
tire rotation, wheel alignment and maintaining proper inflation pressure...and
differences in road characteristics and climate.
Most tires that have wear bars in the tread will indicate when 1/16 inch or less
of tread is remaining. If you see these, it is strongly recommended that you
change the tire. Another way of judging 0.0625 or 1/16 is to use a penny. On a
USA penny 1/16 is roughly from the top of Abe Lincoln's head to the edge of the
The Traction code is a letter(s) AA= Very high, A=High, B=Medium, C=Low
Traction grades represent the tire's ability to stop on wet pavement as measured
under controlled conditions on asphalt and concrete test surfaces. The traction
grades from highest to lowest, are "AA", "A", "B" and "C". A tire graded "AA" may
have relatively better traction performance than a tire graded "A", "B" or "C",
based on straight ahead braking tests. The grades do not reflect the cornering
or turning traction performance of the tires.
These codes are also A, B, and C from high to low. People who drive in the
hot desert should buy "A" rated.
Temperature grades represent the tire's resistance to heat and its ability to
dissipate heat when tested under controlled laboratory test conditions.
Sustained high temperature can cause the tire to degenerate and reduce tire life,
and excessive temperature can lead to sudden tire failure. The temperature grades
from highest to lowest are "A", "B" and "C". The grade "C" corresponds to the
minimum performance required by federal safety standard. Grades "B" and "A"
represent higher levels of performance than the minimum required by law.
The temperature grade is for a tire that is inflated properly and not overloaded.
Excessive speed, underinflation or excessive loading, either separately on in
combination, can cause heat build-up and possible tire failure.
FIGURING OUT YOUR REPLACEMENT TIRE:
Normally, your owner's manual recommends the type of tire for your car, or
it is on a placard somewhere (glovebox for example).
They say use only the same size and construction as originally installed
and with the same or greater load capacity. (or words to that effect). But
people do change their tire size and this is where my story really begins.
You need to know what the manufacturer recommends and the vehicle
capacity weight of your car. An example:
P155SR13 P155/80R13 775lbs/tire for a '87 Toyota Tercel 5 door
The air chamber determines the load capacity and there is a load index
associated with the load capacity. The second number is the load capacity
shown in "pounds" below. The first number is the "index" on the tire using
the new code.
65=639 66=661 67=677 68=694 69=716 70=739 71=761
72=783 73=805 74=827 75=852 76=882 77=908 78=937
79=963 80=992 81=1019 82=1047 83=1074 84=1102 85=1135
86=1168 87=1201 88=1235 89=1279 90=1323 91=1356 92=1389
93=1433 94=1477 95=1521 96=1565 97=1609 98=1653 99=1709
100=1764 101=1819 102=1874 103=1929 104=1984
If you substitute, you must find the aspect ratio that matches or exceeds
the load capacity recommended. So here is an example for P215/75R15 to
/ \ /]-rim-[\------------
( 75 ) 161 mm ( 70 ) 150.5 mm
\_________/________ \_______/ ___________
1742 lbs max 1620 lbs max
(Diagram not to scale, but you get the idea)
Obviously, the air chamber is less and you can not substitute these tires.
If you change the width of the tire, then the aspect ratio will also have
to change to provide the same amount of air space.
The following is my 2 cents on tire classification.
Performance tires are - wide (thicker patch better in corners)
- soft (stick to road whether wet or dry but wear out fast)
- close tread (more road contact)
Snow tires are - narrow (better to cut through snow)
- wide open tread (snow clears out easier)
Long Lasting tires - harder rubber compound (not as good traction)
Any trade off between tires looses you something, either wet and dry performance,
snow performance, or tire life.
When buying tires ask for "fresh tires" and "long valve stems". The fresher the
tires, the better, and long valve stems will make adding air easier.
OK, now that you are an expert ;) go to