A tire sidewall showing its size in metric lists the width first, then the sidewall height (shown as a percentage of the tire width), followed by a letter to designate what type of carcass construction, and lastly the wheel/rim size.
Let’s look at the sidewall of this 245/245ZR17 G-Max AS-03:
This G-Max tire is 245mm wide (about 9.6 inches wide), 25.6 inches tall, and fits a 17-inch wheel. It has a carrying capacity of 1,521 lbs and a maximum operating speed of 168 mph. Note that metric sizing uses both millimeters and inches, and requires conversions. Also, the speed rating references the testing and does not imply that the consumer drive at these high rates of speed.
Certain light truck and heavy-duty tires use American (standard or imperial) measurement in inches. The tire height (or diameter) is most often listed first, followed by the section width, then the carcass construction type, and lastly the wheel size that the tire fits.
Let’s take this 33x12.50R17 Grabber AT2 for example:
For American standard sizing, the tire height, or diameter, is listed directly in inches. On a metric tire, you’ll need to do some math. For metric sizing, take the section width and convert it to inches (divide by 25.4), then multiply that by the aspect ratio (shown as a percentage) to get the sidewall height. Next, take the sidewall height, double it, add the wheel diameter, and you’ll have the diameter of a metric tire.
This is shown in either millimeters or inches, but it refers to the overall carcass width, not the tread width.
Most light truck and passenger vehicle tires built today are built with radial construction (“R”). This is shown before the wheel size. There are three basic types you can find produced today:
Wheel size is shown in inches, regardless if it’s a metric or American sized tire.
The load index shows the tire’s weight carrying capacity. While the load index is a numerical value, it is just a representative symbol for a certain carrying capacity and does not numerically correspond to carrying capacities (Example: Load index 85 indicates a carrying capacity of 1,135 pounds, not 85 pounds or 850 pounds or 8,500 pounds).
Speed ratings are represented by letters ranging from A to Z. Each letter coincides to the maximum speed a tire can sustain under its recommended load capacity. For instance, S is equivalent to a maximum speed of 112 mph. Even though a tire can perform at this speed, General Tire do not advocate exceeding legal speed limits.
Rating Symbol | Speed |
---|---|
P | 93 mph |
Q | 100 mph |
R | 106 mph |
S | 112 mph |
T | 118 mph |
H | 130 mph |
V | 149 mph |
W | 168 mph |
Y | 186 mph |
(Y) | Above 186 |
For tires with a maximum speed rating above 149 mph, a ZR may appear in the size designation. For tires above 186, a ZR must appear in the size designation, and will include a Y speed symbol in brackets.
Prior to (or sometimes following) the tire size—on both American and metric sizing—a tire may or may not have a letter showing the service type. Most consumers will deal with tires stamped with either “P” standing for passenger vehicle tire, or “LT” standing for Light Truck. But there are a number of others.
Tires made for use in the United States are required to have the DOT serial number and date located on one sidewall of the tire near the rim. The sample shows a tire manufactured during the 4th week of 2008. If the date of manufacture is not present on the outside, it will be located on the opposite side of the tire.
Check the pressure in your tires at least monthly and before long trips when your tires are cool (after the vehicle has been stopped 3 hours and then driven less than one mile).
Adjust inflation pressure to the vehicle manufacturer's specified pressure while tires are cold. Never "bleed" or reduce air pressure when tires are hot. It is normal for inflation pressure to build up in your tires as a result of driving.
Use an accurate tire gauge to check pressure and maintain it at the level recommended by the vehicle manufacturer.
Don't forget to gauge inflation pressure in your standard size or temporary spare tire (your temporary spare requires a higher inflation pressure). Remember: under inflation is the most common cause of sudden air loss or sudden failures in any kind of tire.
Tread wear indicators - Cross ribs evenly spaced around the circumference of the tire in the main longitudinal tread grooves which become level with the tread surface when the remaining tread depth is down to approx. 2/32", (1.6 mm).
If stored in unfavourable conditions, or if not properly handled, tires change their physical properties. This can lead to a shorter service life, and the tires may even deteriorate to the extent that they become unusable. Correctly stored and handled tires retain their properties virtually unchanged for several years.
Full size spares, of the same size and type as the road tire, require special maintenance considerations. The spare tire position on the vehicle is frequently not suitable for long term storage for full size spare tires that are not properly maintained /rotated as recommended. Tires contain special ingredients such as waxes, etc. to protect the rubber. These special tire ingredients normally migrate throughout the tire when in service and protect the tire against deterioration caused e.g. by sunlight, humidity and ozone. Therefore, full size spares that are left for prolonged periods of time in the spare tire position should be inspected, properly inflated just like road tires, and included in the regular tire rotation.