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How to Convert Bias-Ply Size to Radial Tire

7/12/2021
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Tags: Tech, Tech
Bias-Ply Tire Sizing Difference
Vintage Firestone Tire

That classic you’ve been restoring is just about done, but it needs new tires. Not a big problem, really. So you wipe the dust off and look for the P number that you are so used to seeing on all the current tires. Problem is, you don’t see it. What you do see, however, is a number that says 700-14. Turns out the last time your car had new tires put on it, they were bias-ply. Bias-ply tires used a slightly different sizing arrangement then the P-Metric scale used today. This article will give you a little help on figuring out what size is closest for your car.

Changing from Bias-Ply to Radial Tire

You can choose to stay original, or you can switch to radial tires. Speedway Motors carries both styles in a vintage look from brands like Coker Tire and Radir Wheels, and you can usually find a direct replacement for your vintage tire. Should you choose to make the change to a radial tire, you can use the chart below to find the closest size to match your original. (Please note: these sizes are as close as we can get, however they are still an approximation.)

Bias Ply to Radial Tire Conversion Chart
P-75 series

(NOTE: On the 83 Bias Ply, there is sometimes a decimal point, i.e. 7.00-14, it varies with manufacturer)

  • So, referring back to our original tire size of 700-14, we can see that an appropriate size that will be close to the original will be either a P195/75/14, or a P205/70/14.

  • To find whether the tire will “look right” or fill the original wheel well like the OEM tire did, you can calculate the overall height of the tire and see if it comes close to the replacement. All you need is the “P-size” of the tire. The formula to do this is as follows:

So with a tire size of P225/50R16…

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