Tires and Your Car: Stuff Classic Car Owners Should Know
Without doubt, the most important and by far the most ignored safety component of any vehicle is its tires. Four contact patches, the surface areas where the tires are in contact with the pavement and comprising about 25 square inches, are the only things controlling how well the car handles, stops and accelerates. That's a lot to ask of our tires, but our vehicle's well being not to mention our own depends upon how well they do their job.
By outward appearance, tires of today look similar to those originally fitted to our classic car. However, almost all passenger tires today are radials, while yesterday's tires were bias-ply. The difference between these two types of tires lies in the design of the casing — the part of the tire underneath the tread that forms the "foundation" of the tire. The casing is made up of a series of cords that are combined to form layers, or plies.
What's the Difference?
In bias-ply tires the layers of cord are positioned so they run at angles to each other across the tire, in a crisscross manner. In these tires the stiff casing was subjected to high friction between the cord layers, which in turn built up high levels of temperature. This greatly increased the rate of tread wear and limited the ability of the tire's sidewall to flex under varying road conditions, in turn limiting the vehicle's handling. Also, these tires tended to "wander" over the center crest of the roads due to their stiff sidewalls.
In radial tires the plies are positioned so the cords run alongside each other in a series of circular bands across the tire. Invented in 1946, radial technology allowed the tire to flex and absorb irregularities in the road surface, while undergoing less overall friction. Longer tread life and increased fuel economy resulted, along with better traction, handling and ride comfort.
Okay, Back to the Point
Almost all cars built before 1972 were fitted with 2 or 4-ply tires. Earlier tires required inner tubes, then later cars that had airtight rims were fitted with tubeless tires. In the U.S. tires were sized according to tread width and rim diameter. For instance, mid-fifties Fords might have come from the factory with 6.00-16 sized tires.
The sizing system back then was simple. The 6.00-16 numbers meant the tire's tread width was 6.00 inches and the rim diameter was 16 inches. If an owner wished to increase the tire size to, say, 6.50-16, he could do so but the tire would not only be wider by 1/2 inch but taller as well, because tire manufacturers kept the same relative aspect ratio (the ratio of the tread width to the tire's height from the rim) to about 80%. The reasons for this included rough roads, stiff suspension systems, and, frankly, habit.
Tire design, coupled with relatively (by today's standards) primitive rubber compounds, limited tread wear to less than 15,000 miles for most cars. Also, hydroplaning was a serious problem in rainy weather and traction in snow was nearly non-existent. Snow tires, studs and chains were wintertime staples for most of the country.
And the Point?
The point we're making here is simple: it'not a good idea to use old technology tires on your old vehicle unless you're a stickler for absolute accuracy; you trailer the thing to shows; it's in a museum; or you have a complete set of spare wheels fitted with radial tires for use on the road.
Old tires weren't very good when they were new and those manufactured from old, original molds are generally no better, due to lack of specialized "tricks of the trade" on the part of the workers making the tires today. In short, these vintage tires look great, but they won't ride or drive like modern tires.
Granted, radial tires aren't offered in exact "look-alike" versions of old tires. Wide whitewalls, redlines, dual whitewalls and other original designs aren't widely available, if at all. Narrow whitewall radials are made in most popular sizes, however, so most 60s cars can be fitted with fairly good copies of original tires.
What to Do?
Hopefully, we've convinced you to use modern radials on your old car if it's driven regularly. Assuming you agree, how do you convert today's radial size to your old car's tire size? Actually, it isn't as hard as you might think, so let's look at how radials are sized and do an example:
All tires today are stamped with an alphanumeric code that is federally mandated to contain certain information about the tire's size compatibility. Also, another stamped area, referred to as the Uniform Tire Quality Grading System (UTQGS), contains key information about treadwear, traction and temperature capabilities.
Looking at a tire you might see the alphanumeric code: P205/70R14 93S. Deciphering the code is easy. The "P" stands for passenger vehicle tires, "205" refers to the overall width of the tire in millimeters, "70" refers to the aspect ratio (ratio of the tire's height to its width), "R" means radial and "14" refers to the wheel rim's diameter. The final number and letter represent the load index and speed rating.
Load index numbers range from 0 to 279 and correspond with the load-carrying capacity of the tire. Most passenger car tires have load indexes from 75 to 105. Our example "93" means the tire can carry a maximum load of 1433 pounds (all tire manufacturers offer load carrying charts, so see your dealer).
The speed rating letter (in this case "S") indicates the range of speeds at which the tire is certified to carry a load. Ratings range from A to Z, with A as the lowest rating, although "H" is out of sequence and refers to 130 mph. A tire with a "C," for example, can only run up to 37 mph. Our "S" tire has a speed rating of 112 mph, meaning it is safe to cruise all day up to that speed, but not over.
Continuing with our example, a small stamped area on the tire might show a Treadwear number of 450, a Traction letter of C and a Temperature letter of A. Treadwear numbers range from 60 to 500, the larger signifying the greatest resistance to wear. Traction letters run from A to C, with the latter having the best all-weather traction. Temperature grades range from A to C, with A signifying the most resistance to heat. These numbers should be used in comparing one tire against another.
Several other important markings appear on the side of a tire, the most important of which are Max Load and Max Press. If a given tire states: Max Load 990 kg (2183 pounds) and Max Press 280 kPa (41 PSI), those numbers mean exactly what they say. The tire will fail if it's overloaded or overpressurized. The most important thing to remember here is the pressure designation: The Max Pressure stamped on a tire is not the pressure it's supposed to be filled to! It is the maximum amount of air the tire can hold without failing. Consult your car's owner's manual or door sticker for the proper everyday inflation pressure.
Back To Our Classic Car
Suppose we want to put radial tires on a '65 Mustang V8. The original bias tire size for this car was 6.00x14, so we need to come close to this size in a radial. The 14-inch wheel size is easy, of course, so we need to convert 6.00 inches (tread width) into metric dimensions. Since 2.54 centimeters equals 1 inch, we need 6.50 times 2.54, or 16.50 centimeters, which in turn equals 165 millimeters.
Now we need to work out the aspect ratio (Remember? The ratio of width to height above rim). Since the original Mustang's tires had about an 80% aspect ratio, we can start looking for radials that are sized close to that specification. Nowadays, 80-aspect ratio tires are relatively uncommon, due to the public's preference for low-profile tires. However, we can find something in a 70 or 75 ratio, so let's choose a P165/75 R14 whitewall for the car. This tire will look very close to original and fill the wheel wells fully, much like the bias ply tires did 37 years ago.
Suppose we don't want quite so much originality? Maybe we want better handling and braking, in addition to a more aggressive stance. In that case we can increase the tread width to 205 mm, for instance, and lower the profile to a 60 series. We'll get better handling because the tire will have a larger contact patch area and its body is narrower. However, there's a price to pay:
The lower profile the tire, the harsher the ride! In addition, lower profile tires exhibit less wet-weather traction and treadwear.
We aren't saying you shouldn't put such tires on your car, just that you need to know the tradeoffs.
Tires are very, very important. Consider carefully how and how often you plan to drive your treasured classic before putting new tires on it. If you do decide to purchase vintage tires, do so from a reputable manufacterer/dealer such as Coker or Lester. Both advertise in Hemmings and both are good at what they do.
If modern radials are your choice you have lots to choose from, so do a little homework and visit some manufacturer websites. You'll be glad you did.