By Glenn Goodspeed (April, 1997)
I know what it's like to go to the tire store when they don't have your size. The last time I went to buy tires for my '63 1800, I asked for stock size Michelins. Guess what? Even Michelin doesn't make the stock size any more. The store had to backorder them from someplace, and I think I may have bought some of the last Michelin XZX tires ever produced.
The most important consideration in buying tires is what you plan to use them for. If your car is a daily driver, you would need a different kind of tire than someone who uses a car for racing, or someone who keeps a car only for concourse showing.
Another factor is driving style. If your style is aggressive, you should consider performance tires and a handgun for backup. My daily driving style is like an aviator's -- I usually don't take chances unless I'm forced to. Once in a blue moon I'll go barnstorming out in the country, where no one will get hurt if I make a mistake, but the rest of the time I meekly tool around the city streets trying to stay out of trouble.
For me, safety is the most important aspect of selecting tires. The safest tires, of course, are tires that won't suddenly loose their ability to hold air, whether from a blowout or by separating from the wheel rim while the car is cornering. They should be carefully manufactured so that they will wear evenly and ride smoothly.
Tires, like many things in life, are a mass of compromises. Expensive tires are built better than cheap tires. Soft rubber sticks to the road better, but it wears quicker than hard rubber. Wide tires give a better grip, but they're harder to balance and control than skinny tires. Tires with reduced tread work better in dry conditions, but they are disastrous in the rain. Low profile tires look cool, but they don't provide the shock-absorbing benefits of thicker tires, which can prevent damage to suspension components as well as your back.
I got a good laugh (up my sleeve) last spring making small talk with a new business contact. I complimented her brand new Pontiac Firebird convertible that had all the fancy options -- big V-8, beefy suspension, dual exhausts, and high-performance tires. She was in love with the car, and had been heartbroken when a month earlier she slid off an icy road and hit part of a bridge. She was going very slow and the damage was minor, but she was mystified as to why her spiffy new car couldn't hold the road.
I looked at the tires. "Here's the problem," I told her. At temperatures under about 50 degrees Fahrenheit, high-performance rubber looses its elasticity just like putting bubble gum in the fridge. Add to that the reduced tread, and she probably should stay home on a cold, wet day.
This is why manufacturers grade tires by purpose. Once you figure out what your purpose is, you can narrow down the choices quickly. All-season tires have the widest range of applications. I would recommend them for anyone who doesn't need snow, mud or high-performance tires. Not only are high-performance tires limited in their applications, neither do they provide a significant advantage in daily use, unless looking cool is significant to you.
Now that the proper tire sizes are becoming more difficult to obtain, P1800 owners are having to try substitutes. You can still find cheap tires in the correct sizes without too much trouble. This is unfortunate, because some owners are wasting money on tires that are unreliable, short-lived and hard to balance, because they don't realize that suitable high-quality tires can be substituted.
I try to stick with stock tire sizes, because they always fit right, and they don't mess up the speedometer readings or the fender wells. If I must substitute another size, I try to use the closest possible match with the stock size. Sometimes it's just a matter of finding a tire store manager that will take the extra time to call suppliers.
Some difficulty finding good tire substitutes arises from the fact that there are at least three popular systems of sizing tires. The system used by Volvo and other European manufacturers in the sixties and early seventies included the tire width in millimeters, a speed rating, the type of construction, and the wheel diameter in inches. For instance, the stock tire size on my '63 is 165SR15. This tells me that the tire width is 165mm, it is dependable at sustained speeds up to 112 mph, has radial construction, and fits a 15-inch wheel.
The newer P-metric sytem of measuring tire sizes includes the tire width in millimeters, speed rating, construction type, aspect ratio, and wheel diameter. So a P195/60SR15 tire would be 195mm wide and its height from wheel rim to crown would be 60 percent of its width. (This is the aspect ratio.) The other designators are the same as in the old system.
As you can see, the only difference between the old system and the new one is the addition of the aspect ratio. To compare the old tire sizes with the new, assume the aspect ratio on the old size is about 75. The aspect ratio describes the profile of the tire. A tire with a very low aspect ratio (50, for instance) has a low profile, meaning that, viewed from the side, it looks like there's not much rubber between the rim and the road.
Aspect ratio never used to be important, because manufacturers only made tires with an aspect ratio of about 75. Since tire technology and consumer tastes have changed, the new system has become better for describing the new tires.
Many tires sold nowadays do not have a speed rating. Since speed ratings start at over 100 miles per hour, I would not worry about this unless you live in the boonies and really plan to do a lot of driving at sustained high speeds. In case you're worried about loosing the cops next time you rob a bank, most good-quality tires probably can handle speeds over 100 mph for a few minutes without problems.