Re-tiring

by Robert Pontiff

Since the first Thunderbird, tire size conventions have changed a number times and the tire construction method has also changed. The radial-ply tires have replaced bias-ply tires.

There are companies that specialize in providing "vintage" tires. Many judging organizations require that you use original brand, style and size tires. These vintage tires tend to be expensive and obtaining them lacks the convenience of popping down to the local Walmart or Goodyear Store. If originality is not an issue, then a number of benefits can be derived from using "new-style" tires. A "new-style" tire may be safer, less expensive, more readily available and last longer.

The use of radial tires versus bias-ply tires requires no major modifications to your automobile. Some suggest that radial tires be inflated to 3 PSI higher than normal to compensate for the less-stiff sidewall. Adjusting the front-end alignment toe-out slightly outside of specification also helps "the feel" that is resultant from the use of radials. Do not use radial tires with tubes. All Thunderbirds were delivered with tubeless tires except for those with Kelsey Hays wire wheels (1962-1964).

The tire-size convention used in the 1950's through mid-1960's is represented by "6.70-15" or "670-15". The first three digits ("670" or "6.70") of this code provided the approximate width of the tire in inches. "6.70" is 6.7 inches. The last two digits ("15") provide the rim diameter in inches.

After the mid-1960's the tire-size convention is represented by "LR60-15". The "L" is an alpha letter designation and defines load limit. "R" means that the tire has radial-ply construction. If the tire designation has only one letter e.g., no "R" then the tire is bias-ply. "60" provides the aspect ratio. The aspect ratio is the height of the tire (from top of rim bead to bottom of tread) divided by the width of the tire. The "15" represents the rim diameter in inches as above.

The current tire-size convention is a variation an original metric system and is represented by "P215/75R15". "P" means passenger car tire. (The original metric system omitted this letter.) "215" is the width of the tire in millimeters where "215" is 215 millimeters. "75" represents the aspect ratio as defined in the earlier convention. . (The original metric system omitted aspect ratio.) The tire construction is represented by "R" and can be "R" for radial-ply, "D" for diagonal-ply or "B" for bias-ply construction. As in previous conventions "15" represents the rim diameter in inches. This convention was used for 1980 and up Thunderbird tires.

The following is provided as a guide in determining replacement tire sizes:

T-bird model Original tire size Suggested Replacements
1955-1956 6.70-15 P195/75R15 or P205/70R15
1957 7.50-14 P215/75R14 or P225/70R14
1958-1964 8.00-14 P225/75R14 or P235/70R14
1958-1960 8.50-14 P165/75R15 or P175/70R15
1964-69 8.15-15 P205/75R15 or P215/70R15
1968-69 8.45-15 P215/75R15 or P225/70R15
1968-73 215R15 P215/75R15 or P225/70R15
1970-71 H78-15 P215/75R15 or P225/70R15
1974-75 LR78-15 P235/75R15 or P255/70R15
1976 JR78-15 P225/75R15 or P235/70R15
1977-78 HR78-15 P215/75R15 or P225/70R15
1979 GR78-15 P205/75R15 or P215/70R15