Tire sizes explained

Written by kyle mcbride
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Tire sizes explained
(Allyn Bowker)

Vehicle tires are much more complex now than in years past. In response to the widely diverse requirements of vehicles today, tires need to conform to certain specifications. These specifications include size, load and speed, and they must physically fit on the intended vehicle. The passenger tyre's size is moulded into the sidewall and given in the P metric method of sizing, which was established in 1976. The tire size may read like some secret, technical code, but it is, in fact, very informative.

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Tire Type

The tire type indicates the intended use of the tire. The letter P indicates it is a P metric tire and its proper intended use is on passenger cars. The letters LT indicate it is an LT metric tire and it is for use on light trucks. P metric tires and LT metric tires are not interchangeable.

There are speciality tires that will have other tire type designations. The letters ST indicate the tire is a trailer tire. The letter T indicates it is a temporary tire. An example of the temporary tire would be the space-saving mini spare tire that most passengers cars have in the boot.

Tire Size

This number indicates the nominal width of the P metric tire. The nominal width is the width of the tire in millimetres from sidewall to sidewall as measured when the tire is mounted on a rim and properly inflated but not supporting the weight of a vehicle.

Aspect Ratio

This is the ratio of the height of the P metric tire from tread to rim to its nominal width. An aspect ratio of 65 means that the tyre's height from tread to rim is 65 per cent of the tyre's nominal width. The aspect ratio is also referred to as the tyre's profile.

Construction

This letter indicates how the P metric tire was constructed. The letter R indicates that the tire is a radial. The letter B indicates that the tire is a cross-ply. The letter D indicates it is a diagonal ply tire.

Wheel Diameter

The wheel diameter is the width of the middle of the tire to accommodate mounting on a wheel. It is measured from bead to bead across the opening of the centre of the tire. Whereas the tire size is in metrics, this dimension is in inches. The number 15 indicates that the tire should be mounted on a 15-inch wheel.

Load Index

The load index is the maximum load that a P metric tire can support when it is inflated to its proper tire pressure. The load index number is a two- or three-digit code between 60 and 125 that represents the rated weight in pounds. On the example tire in the opening diagram, the load index of 89 indicates that the tire is rated to carry 580 Kilogram. Check the manufacturer's website for a chart that shows the codes and corresponding weight limits.

Speed Rating

The speed rating is a one- or two-character code that indicates the maximum rated speed that the tire can sustain over 10 minutes and still be considered operating safely. The speed rating of H that is on the example tire shows that it can safely attain a speed of 130 miles per hour. In most normal driving situations, the maximum speed rating for a tire will not be tested and it is not advisable to do so. Obey local speed limit laws. Check the manufacturer's website for a chart that shows the codes and corresponding speed limits.

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