American Tires ... What The Numbers On The Sidewalls Mean

Related Topic:
Calculating Tire Dimensions

I have been a certified mechanic since I was an 18 year old senior in high school, and since 1981, I have been an auto mechanic, primarily employed in tire shops. I have mounted and balanced literally tens of thousands of tires in my career, and have accrued quite a bit of knowledge in this area. 

ARE YOU READY TO RE-TIRE?
Your new car is not so new any more. The mountains and valleys of tread are looking more like a glassy inland lake, smooth and shiny. It is now time to bite the bullet, or in this case the wallet, and head down to the tire shop for some new skins. Before you just take the word of the sales person on which set to buy, read and contemplate this information first.

In choosing tires for your car the first thing you want to ask yourself is under what conditions will most of my driving be in. If you live in an area where it is sunny most of the year, you will probably choose a straight ribbed highway
tread with mostly closed shoulders and a very close pattern. You won't need to worry too much about snow or ice, and an all season radial would not be necessary.

If you live in an area where the weather changes with the seasons, and you will run into snow and ice enough of the year where it will warrant an all season tire, by all means this would be the most intelligent way to go. 

And obviously if you live among the penguins in Antarctica, or in Canada and the snow belt area's like I do, then possibly snow tires added into the mix would be in order. Trucks will be a different story all together. If you are replacing truck or SUV tires, the more time you spend off road, the more aggressive the tread should be for better traction. If this is primarily a road driven vehicle, an all season or highway treaded tire would give you much better service.

CONTRARY TO WHAT SHE SAYS, SIZE DOES MATTER
On the inner door jam of your vehicle there will be a sticker that will tell you the recommended size tire the car should have on it. This is the size you should stay with, or at least a tire of the same diameter as this. In many states it is unlawful to put a smaller size than the manufacturer suggests on the car at replacement time because the load carrying capacity is less than it should be, and you may risk tire damage, a blowout, or even an accident. Should the unthinkable occur, and injury results, you could be held liable for damages, or even someone else's life.

This is a quick rundown of what those numbers on the sidewalls mean.

For illustration sake we'll take a tire size of P 225/75R15. The "P" stands for passenger, or just regular tire, not light truck or anything. The first number is the width of the tire's cross section in millimeters, from side to side, so in this instance it means 225 millimeters. The second number means the aspect ratio between width to height, so the example tire is 75% as high as it is wide. Thirdly the 15 simply means it is a 15" tire. On a truck or SUV, if it came with LT series tires, or "light truck" you will want to replace them with a rating equal to this.

If you see a car, for instance with the middle number lower than 70, it is more of a high performance tire. We'll choose a P 225/60R15 for instance. The 60 means it is only 60% as high as it is wide, so the sidewalls are shorter than the 75 series tire. The smaller the middle number the shorter the sidewall, and stiffer the tire will be. It will flex less, because it is designed for higher performance driving than a standard tire. And as a higher performance tire it's construction is different and it will be more expensive to replace once it is worn.

In keeping with the same size you run into no legal problems, but more importantly everything in your car will work like it was designed to. If you over size tires, (buy a bigger tire), this is not against any law I am aware of, but it may mess up the speedometer and odometer of your car or truck/ SUV and possibly even some computer controlled equipment. So if this is your choice, check with the vehicle manufacturer to see if any problems will result from this endeavor.

AHHHH, MORE NUMBERS
Some other numbers you will see in the tire brochures are Treadwear, Traction, Temperature, and Speed Rating. These numbers are simply government standards which are for the most part a revelation on the quality and life expectancy of the tire.

Tread Wear:
More Than 100 - Better 
100 - Baseline 
Less Than 100 - Poorer

The tread wear grade is a comparative rating based on the wear rate of the tire when tested under controlled conditions on a specified government test track. A tire graded 200 would wear twice as long on the government test track as one graded 100. There are plenty of tires with tread wear ratings of over 500 even up to almost a thousand. These obviously would be very long lasting, but also very expensive.

Michelin, for instance, has some very high numbered tires in tread wear, and some very high sales receipts to show for it. One caveat though, in choosing a high tread wear number, the rubber compound gets very hard, and may not provide adequate traction in extremely cold climates, so choose wisely my higher longitudinal friends.

I installed a set of tires made in Israel a few years back with a tread wear of 65. These tires were so soft they almost wore out rolling them across the shop floor. I think they were made from old pencil erasers or something, I don't think you want to get any 65's.

Traction:
A - Best 
B - Intermediate 
C - Acceptable

Traction grades represent the tire's ability to stop on wet pavement as measured under controlled conditions on specified government test surfaces of asphalt and concrete. The Traction grade is based upon "straight ahead" braking tests, it does not indicate cornering ability.

Temperature:
A - Best. 
B - Intermediate 
C - Acceptable

Speed Rating:
The speeds associated with the numerical part of these following numbers are understood to be the maximum allowable sustained speed.

S = 112 M.P.H.
T = 118 M.P.H.
U = 124 M.P.H.
H = 130 M.P.H.
V = 149 M.P.H.
W = 168 M.P.H.
Y = 186 M.P.H.
Z = Above 186 M.P.H.*

* For tires having a maximum speed capability above 149 M.P.H. a "ZR" may appear in the size designation. For a tire having a maximum speed capability above 186 M.P.H. a "ZR" must appear in the size designation.

There are some other numbers as well. Like load index and others, but these are not so important that I need to bore you with them right now. If you own a high performance car where you would need to know if it required a "Z" speed rated tire, chanced are you are fully educated in this already.

NOW GET ON YOUR CELL PHONE AND YELL, BUY BUY BUY
This should get you well on your way to being able to look for your new tires. Buy the highest rated tire you can afford, and the best type depending on the conditions you spend most of your time driving in.

Be confident in your purchase by buying from a dependable and reputable source.

Saving Money
To save a lot of money on those new tires once you know what's right for you ... I recommend you try this reputable source

Don't get caught up in any of the "my brand can beat up your brand" mentality, because if the tires are from a major manufacturer, they will carry a good warranty and will be sold across the country should you have a problem on the road some where.

Stay away from regional brands, and no name brands no matter how good the deal sounds.

When shopping for tires in large discount retails stores, be careful. Here's an interesting story to illustrate the point.

Keep the air pressure at the recommended P.S.I. on the sidewall of the tire. Keep the front end in align, and your rubber should serve you well.

article courteousy The Tire Rack

Related Topic:
Calculating Tire Dimensions