Radial and Bias-Ply Tire Markings: How to Tell Them Apart

The short answer

Radial and bias-ply tires differ in how the internal cord layers, called plies, are laid inside the tire body. You can usually tell which type you have from the letter between the width and rim-diameter figures in the size code. An "R" means radial construction. A "B" or a plain dash means bias-ply construction. For example, a size written as 6.00-12 or 6.00B12 identifies a bias-ply tire.

Two conditions change how far that answer gets you. First, the marking identifies construction only. It does not tell you whether a tire suits your vehicle, trailer or load. Second, a radial and a bias-ply tire that look alike in size are not automatically interchangeable. Fitment, load capacity and inflation figures must come from the vehicle maker, the trailer maker or the tire maker.

What "radial" and "bias-ply" mean

A ply is a layer of reinforcing cord embedded in the rubber of the tire's carcass, the structural body under the tread.

• Bias-ply (also called cross-ply or diagonal): the plies run at an angle across the tire, with successive layers crossing each other in a crisscross pattern.
• Radial: the plies run straight across the tire from bead to bead, at right angles to the direction of travel. The bead is the edge that seals against the wheel rim. Separate belts under the tread reinforce the crown.

Construction affects how the tire behaves. Comparison material from tire and trailer sellers describes the radial design as keeping the contact patch, the area where rubber meets the road, from deforming under load the way a bias-ply tire's does. One tire retailer's comparison describes bias tires as durable, puncture-resistant and typically more affordable than radials. Treat these as general claims from those sources. Confirm the performance of any specific tire with its manufacturer.

Reading the construction letter in a size code

The construction indicator sits between the section-width figure and the rim-diameter figure. The general pattern is shown below with placeholders, not real sizes:

[width]R[rim diameter]   ->  radial
[width]B[rim diameter]   ->  bias-ply
[width]-[rim diameter]   ->  bias-ply (dash form)

Examples of bias-ply notation:
6.00-12
6.00B12

Size codes can carry more information than this, such as a service-type prefix, an aspect ratio, a load index or a speed rating, depending on the tire category and market. Those elements are outside the scope of this guide. Use the tire maker's documentation or an applicable standards reference to interpret them.

Marking | Construction | Example

R between width and rim size | Radial | Pattern: [width]R[rim]

B between width and rim size | Bias-ply | 6.00B12

Dash between width and rim size | Bias-ply | 6.00-12

Converting between bias-ply and radial sizes

Older vehicles often list bias-ply sizes. Many current replacements are sold in radial and P-metric sizes. Cross-reference charts exist to map one to the other, and they typically start from the tire's "P-size."

Gap: no chart values were supplied for this guide, so no specific conversions are given here. Equivalent sizes in a chart are approximate matches in dimension. They do not confirm that load capacity, overall diameter, wheel compatibility or clearance are suitable for a given vehicle. Check any conversion against the tire maker's published specifications and the vehicle's requirements before buying.

Trailer tires

Trailer tires come in both constructions and use the same R, B or dash convention. Trailer-focused sources compare the two mainly on how the tire holds its shape under load. Gap: no load ratings, inflation figures or trailer-specific standards were supplied. Take those from the trailer maker, the tire maker and the tire's own sidewall information.

Classic cars and mixing constructions

Owners of older vehicles often ask whether to use radials or bias-ply tires. Some also ask whether one type can go on the front and the other on the rear. Enthusiast discussions show that preferences vary: some owners run radials all round, and others report mixing. That is anecdotal preference, not guidance.

This guide does not recommend mixing constructions. Get any decision about combining radial and bias-ply tires from the vehicle and tire manufacturers. One classic-car owners' resource also notes that radials can mask worn steering linkage and suspension components. If you are converting an older car to radials, that is a reason to have those parts inspected rather than assume the new tires reflect their condition.

Limits of this guide

This guide explains construction terms and size notation only. It is not fitment approval for any vehicle, trailer or wheel. It does not provide load, pressure, speed or legal requirements. Those depend on the specific tire, the vehicle or trailer, and the jurisdiction, so verify them with the relevant manufacturer or regulatory documentation.