How to Convert Bias Ply Tire Sizes to Metricby John Walker
Bias-ply tires date back to 1898. With the development of vulcanized rubber, Goodyear began creating bias-ply tires with cords of fabric built into the rubber for stability. The sizing on bias-ply tires may seem a bit odd, as it is largely based upon wheel-diameter. Most newer passenger vehicles use a P-metric system (such as P235/75R15) which is based on aspect ratios, side-wall heights, and rim diameters. Converting bias-ply to P-metric tire sizing is not difficult.
Identify the bias-ply tire's diameter. The bias-ply measurement is printed on the original tire or in the original specifications for the vehicle. Locate this measurement on the tire size chart included in the Resources section, and the diameter that is displayed next to the measurement size. For example, the A78-14 tire has a 24.2-inch diameter. If you are unable to locate the written measurement of your bias-ply tire, measure the tire from top to bottom to get the diameter.
Cross-reference the tire's diameter with the conversion chart included in the Resources section. Locate the tire's diameter and to the right of that measurement will be a P-metric size close in dimensions to the bias-ply. There will be very few exact matches, as the construction of the two types of tires results in variations on height and width.
Locate P-metric sizes that are close to the diameter of the bias-ply tire if no exact matches are listed. You may have to choose one that is right above or right below the diameter. For example, a 25.8-inch tire does not have an exact match, but the P215/70R14, P185/75R15, and P195/70R15 are the closest matches. Multiple measurements as possible matches means you will compare multiple sizes of tires.
Input the P-Metric size or sizes into the tire size calculator included in the Resources section. The calculator will provide the dimensions of the P-metric tire, while the tire size chart will provide you the dimensions of the bias-ply. The calculator compares two P-metric sizes so you can input two at once.
Compare the revolutions-per-mile of the P-metric tires to the listed revolutions-per-mile printed on the tire chart showing the bias-ply measurements. On both the chart and the calculator, revolutions-per-mile is indicated by "Rev / mile." Keep the revolutions-per-mile of the P-metric tire as close as possible to the bias-ply measurement. For example, 889 revolutions-per-mile is relatively close to 897 revolutions-per-mile.
- In P-metric measurements, the three-digit number following the P is the tire's tread width. The larger the number means the wider the tire. If you are replacing a thin tire (many bias-ply tires were narrow), choose a lower P- size.
- If the revolutions-per-mile of your new and old tire are too far off, you can negatively impact the performance of the vehicle, including the potential for brake failure or a possibility of a speeding ticket as your speed listed on the speedometer will be inaccurate.
- tire image by sasha from Fotolia.com