Tire Size

Important: Similar-looking tire sizes are not automatically interchangeable. Overall diameter, section width, recommended rim width, load capacity, vehicle clearance, and other fitment factors should be considered when comparing tire sizes.

How do I find the correct tire size for my classic car?

Start by identifying the original tire size specified for your vehicle's year, make, model, and configuration. Period owner's manuals, factory specifications, existing fitment information, and historical tire references can all be useful starting points. Once the original size is known, you can research tires available in that size or compare appropriate alternatives.

When choosing a replacement, consider more than the size designation. Overall diameter, section width, recommended rim width, load capacity, tire construction, and available clearance can all affect fitment. Vehicles that have been modified or fitted with non-original wheels may require additional measurements before a tire is selected.

What do old tire sizes such as 6.70-15 or 670-15 mean?

A traditional numeric tire size such as 6.70-15 uses an older sizing system commonly found on classic vehicles. The 6.70 portion refers to the tire's nominal section width in inches, while 15 refers to the diameter of the wheel in inches. The same general size may also appear as 670-15 in some tire listings and historical references.

Numeric sizes were widely associated with bias ply tires before newer alphanumeric and metric sizing systems became common. Because historical tire dimensions can vary by manufacturer and design, the complete tire specifications should be reviewed when comparing replacements.

What is the difference between a bias ply tire and a radial tire?

Bias ply tires are built with cords that run diagonally across the tire from bead to bead, creating a crisscross structure. Radial tires are built with cords that run perpendicular to the direction of travel, usually with reinforcing belts under the tread. These construction differences affect ride quality, handling, tread wear, heat buildup, and overall driving character.

Many classic vehicles originally used bias ply tires, while modern passenger vehicles typically use radials. Owners of classic cars may choose bias ply tires for a more traditional feel and appearance, or they may consider radial alternatives when looking for different handling and wear characteristics.

Are vintage and modern tire sizes directly interchangeable?

Not always. A vintage size and a modern size may look similar on paper, but that does not mean they are automatically interchangeable. Overall diameter, section width, rim width compatibility, load capacity, and clearance all need to be compared before making a change.

Even when a modern radial is offered as an approximate equivalent for a classic bias ply size, the two tires can still differ in profile, contact patch, and ride behavior. Reviewing the full specifications for each tire is the safest way to evaluate whether a substitution is appropriate for a specific vehicle.

Why does the same classic vehicle sometimes show more than one tire size?

The same classic vehicle can be associated with more than one tire size because factory fitment often varied by year, model, trim, body style, wheel option, or market. Later owners may also have switched to replacement sizes, radial equivalents, or different wheel diameters over time.

For that reason, tire listings for a classic vehicle should be treated as a starting point rather than a single permanent answer. Confirming the vehicle's exact configuration and current wheel setup is the best way to narrow the correct size.

What is an alphanumeric tire size such as F78-15?

Alphanumeric tire sizes combine a letter and numbers to describe the tire's size classification and wheel fitment. In a size such as F78-15, the letter identifies a size or load category, 78 represents the tire's aspect-ratio family, and 15 indicates the wheel diameter in inches.

This sizing system became common on many American vehicles during the 1960s and 1970s. When replacing an alphanumeric tire size, it is helpful to compare its actual dimensions with both period-correct tires and any modern alternatives being considered.

What is the modern radial equivalent of a classic tire size?

A classic bias ply size may have one or more modern radial sizes with broadly comparable dimensions, but there is not always a single exact equivalent. A suitable radial alternative should be evaluated by comparing overall diameter, section width, wheel diameter, recommended rim width, load capacity, and available vehicle clearance.

Some classic applications may also have bias-look radial options, which combine radial construction with proportions and sidewall styling designed to complement vintage vehicles. Always review the specifications for the exact tire rather than relying only on a conversion chart.

Can I replace bias ply tires with radial tires on a classic car?

Radial tires are available for many classic-car applications, but changing from bias ply to radial construction requires careful size and fitment consideration. The replacement should provide suitable overall diameter, width, load capacity, wheel compatibility, and clearance for the specific vehicle.

Tire construction can also influence steering response, ride characteristics, and the appearance of the vehicle. Owners prioritizing factory-correct restoration may prefer bias ply tires, while drivers looking for radial construction can research standard radial or bias-look radial options designed for classic applications.