What Tire Sizes Are Interchangeable?by Margo Dill
Tires sold in the United States must meet certain standards. They have to meet size standards for bead shape, diameter and width. The U.S. Tire and Rim Association and the European Tire and Rim Technical Organization also agree on other parameters of the tires. These standards, set by these organizations, are usually interchangeable. There are still several tire size systems that are in use today. The system that your automobile uses may depend on when your vehicle was made. Because automobile performance increased and vehicles became more complex over the years, tire size systems have also changed. There are some tire sizes expressed with only numerals and others with letters and numerals.
Numeric and Alphanumeric Systems
The numeric tire sizing system is the oldest in the United States. It is used for passenger car tires. When this system was used, tire aspect ratios were either 82 or 92. The aspect ratio is the relationship between the tire's heighth and width. The height is measured from the inside of the tire to the outside wall when the tire is fully inflated. The width is also measured when the tire is fully inflated from one stonewall across to the other side. For example, a tire that had the numeric tire size of 7.00-14 has a section width of 7 inches, a rim diameter of 14 inches, and an aspect ratio of 92.
With the alphanumeric system, which was established in 1968, the tire size is based also on the tire's load carrying capacity in relation to its overall size. The tire's capacity and size are indicated by letters from A to N. A tire marked with A would mean the smallest tire with the lowest capacity. A tire marked with N would be the largest tire with the highest capacity. BR78-13 would be an example of a tire size in the alphanumeric system; 78 is the aspect ratio and 13 is the wheel size in inches.
Today's Tire Size Systems
Tire size systems today depend on many things. The number and letters will indicate when a vehicle was manufactured and whether it was produced domestically or imported. The tire sizing system also provides information about a tire's dimensions. The three most important measurements to get a proper fit for your vehicle's tires are height, width, and load carrying capacity. The P-Metric tire size system is the United States version of the European metric sizing system, which was started in 1976. P-Metric sizes always start with P, which stands for passenger. Otherwise, P-Metric and Metric are the same.
An example of a P-Metric tire size would be P215/65R15 89H. The P stands for passenger, of course. The numeral 215 is for the tire width in millimeters. The number 65 is the aspect ratio of this tire. The R stands for radial, as opposed to diagonal or belted tires. The 15 stands for inches, not millimeters, and it is the measurement of the rim diameter. Finally 89H is the service description. The 89 stands for the load carrying capacity, and the H is for the maximum speed for which the tire is rated.
If you have an older car which has tires that are written in numeric sizes, then you will need to convert those to P-Metric sizes to find tires for your car. Early cars had narrow tires and narrow wheels. Today's tires, with P-Metric sizes are usually talked about as a series, according to their aspect ratios. For example, a car that has a tire with an aspect ratio of 80 is known as an 80-series. If you have an early-made car with a numeric size tire, then you will want a tire that is today's 75- or 80-series size tire. This is really important if the vehicle still has the original wheels.
If you have a vehicle with an alphanumeric size, then to find interchangeable sizes in today's systems, you need to identify your tire's aspect ratio. If you have a 78-series alphanumeric tire, it should be replaced with today's 80 or 75-series tires. Again, you are looking for the more narrow tire as in the above examples. If your alphanumeric size tire is a 50, 60 or 70-series tire, then you will need to find a P-Metric tire size that has an equivalent aspect ratio. You can do this with a conversion chart or by reading the P-Metric tire size. Remember, the aspect ratio is the 2-digit number after the slash mark on the P-Metric tire size. In the example in Section 2, P215/65R15 89H, the aspect ratio would be 65, and this would match with an alphanumeric size tire that had an aspect ratio in the 60-series.
Never mix radial tires with any other construction on a car. Radial tires could be used only if the wheels are replaced on your older car and the suspension was adjusted. You should also never mix radial and bias tires on the same vehicle at the same time. Radial tires are not interchangeable with bias tires!
Tire Conversion Charts
If you own an older car, it will be important for you to know about your tires and what sizes are interchangeable. You can ask at your dealership. Most dealers have a tire conversion chart that will help you select the correct tires. You may also be able to find some tire conversion charts on the Internet, however, your dealer's may be more reliable. You can also figure out if the conversion chart is leading you in the correct direction to find interchangeable tire sizes. It is important to understand what all those letters and numbers mean on the side of your tires to be able to find the correct tires for your vehicle.
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