Mustang MonthlyHow To Wheels Tires
Choose The Right Tires
Making the most of where the rubber meets the road
Tire sizing can be confusing. If your '65 Mustang was designed for 6.95x14 tires, what do you install on it now? In 1965, tire sizing was based on section width and wheel diameter. A 6.95x14 tire was 6.95 inches wide (sidewall to sidewall) around a 14-inch-diameter rim. A 7.00x13 tire was 7.00 inches wide around a 13-inch-diameter rim, and so on.
Beginning in 1968, tire manufacturers went to an alphanumeric system of tire sizing that started at the beginning of the alphabet with the smallest tire size. For example, a C78x14 tire was the direct replacement for a 6.95x14 tire. The D78x14 tire replaced the 7.35x14 tire size. The E78x14 became the replacement for the 7.50x14, the F78x14 replaced the 7.75x14, and so on. None of these sizes was an exact replacement, as there was some variation. The two-digit number was the aspect ratio, which is the tire's height (tread to bead) to the section width. This means a 78-series tire is wider than an 84-series tire. That said, a 60-series tire is wider than the 70, 78, or 84.
When radial tires came along in the late '60s, the alphanumeric system of sizing took on the letter R to indicate a radial tire. For example, an F78x14 became an FR78x14.
During the '80s, tire sizing changed yet again with the P-Metric system of tire measurement. Using the example P195/78R14, P means passenger car. Next, 195 means section width in millimeters. The 78 is the aspect ratio, just like the 78-series tires. R means radial construction, and the 14 means rim diameter in inches.
In more recent years, tire manufacturers have gone to the ISO (International Standard Organization) Metric system of tire sizing. It's similar to the P-Metric system originally used in Europe. The ISO system sizes tires the same way as P-Metric, but with the addition of load index and speed ratings after the tire size. You'll see 195/78R14 82H. The two-digit 82 is the load index, or how much weight the tire is designed to carry. The letter H indicates the speed rating, or how fast the tire is designed to go.
What does all this mean when trying to properly size P-metric and ISO Metric radial tires to a classic Mustang? Your Mustang's original 6.95x14 tire size is easily converted to P-metric sizing. First, the aspect ratio of your Mustang's 6.95x14 tire sizing is around 83. All we have to do now is convert 195 millimeters to inches. One inch amounts to 25.4 millimeters. With that in mind, 195 millimeters is 7.68 inches. This is nearly three quarters of an inch wider than the 6.95-inch width of our Mustang's original tires.
The next P-metric size down--which is 175 or 175 millimeters--amounts to 6.89 inches, which isn't wide enough. So, stick with the 195s for best results. This works when we want to install P-metric whitewall radial tires on our classic Mustangs. We have to get it as close as we can, even if it means going a tad larger.
Through the years, we've learned that P195/78R14 and P205/78R14 are ideal Mustang tire sizes if you're seeking original outside-diameter tire sizing. This keeps tire height close to original and your speedometer reading close to accurate. The P195 is closest when it comes to speedometer accuracy.
Performance Mustangs, like the Boss 302, 429, and 351, perform best with P215 or P225/60R15s. Mach 1s and GTs originally fitted with E70 or F70x14 tires will live happily with P215 or P225/70R14s or 15s. It's a good idea to fit-check any tire you're thinking about buying before committing to the expense.