The Plus One
The Plus concept takes it a step further, and formulates the tire size you'll need if you increase your wheel diameter by an inch, for instance changing from a 15-inch to a 16-inch wheel and tire. As the wheel diameter goes up, the aspect ratio goes down to compensate for the changes. There are also formulas for Plus Two, Plus Three and so on depending on how big you want to go.

You can even do a Plus Two or Plus Three, as long as you keep the dimensions under control. For a cool way to see the difference your tire size choices make, and to calculate what your options are, check out this Plus Sizing Conversion Calculator from the able minds at Miata.net.

The government wants us to be safe, so they're making sure that if you're going 125 mph you have the right tires on your car. Through actual testing of the tire, they rate each one according to how fast you can safely travel on a set of them. It's pretty straightforward when you check the chart. Each letter represents a maximum speed. Chances are your tires will be rated for much higher speeds than you'll ever go, but it's still a useful number. The faster a tire can go, the softer the rubber compound they use to make it (softer rubber grips dry pavement better), so the tire will wear out faster than a "slower" rated tire.

Speed Ratings
Q- Up to 100mph
R- Up to 106mph
S- Up to 112mph
T- Up to 118mph
U- Up to 124mph
H- Up to 130mph
V- Up to 149mph
W- Up to 168mph
Y- Up to 186mph
Z- 149mph and over

 

There is so much room for adjustment in terms of size, it would take an encyclopedia to list them all. Luckily, if you're visiting  you're smarter than the average tailgater, so I'll give you the tools to decide for yourself.

Tire Width
A wider tire will give your car better grip on dry pavement, but your gas mileage will suffer. Too wide and your tires may rub your car's body, destroying the tire. Narrow tires are better for traction in winter weather. But go too narrow and you're creating a safety risk. Your car relies on how much rubber touches the road to keep you going in the right direction. Unless you're trying to do some serious experimenting or you are being advised by a pro, you should always go with the size tire the auto manufacturer suggests.

 

Aspect Ratio You learned back in the Tire Ratings intro that a tire's aspect ratio determines how tall the sidewall is.

 

A lower, stiffer sidewall will greatly improve cornering, but will make your car's ride a little harsher. That's why a Cadillac has a nice big bubble tire while a Porsche looks like somebody put a black rubber band around the rim. Changing the aspect ratio too much can throw your speedometer off, which is why some people go a a [url link=]Plus-One[/link] setup. This way you have the benefit of solid cornering and you'll know how fast you're going.

Wheel Diameter
This is pretty much non-negotiable. If your car has 16-inch wheels, you'll need 16-inch tires. The only way to change this is to buy new wheels, for instance if you were going with a Plus-One or even a Plus-Two setup.

 

Once you've figured out what all the gobbledygook on the side of your tires means, you're probably asking yourself, "Do I really give a hoot?" Believe it or not, that's a good question! Some of the info will certainly apply to you, but some of it can be flushed out of your consciousness without worry.

What Matters to Me?

The Size
Obviously, the size is the single most important piece of info on the tire - get it wrong and it might not even fit. Size includes the width, aspect ratio and wheel diameter. These three measurements will be universal no matter what brand of tire you're looking at. There's a lot of room for experimentation and error in this realm, so tread with care. Be sure you know what happens when you change sizes before you deviate from the standard.

Speed Rating
The average driver doesn't need to be too concerned with a tire's speed rating.

 

There aren't any tires out there that will blow at 70 mph. However, there are tires that will blow at 120 mph, so if you are planning on driving at those speeds, you better pay attention to this number.

Loads and Pressures
These ratings aren't going to be too important to you, either, but there are a few interesting points of information here. For instance, if you are comparing two tires that are the exact same size, but one of them has a higher maximum tire pressure, this tire may get slightly better gas mileage since a firmer tire offers less resistance. Your load ratings will only be important for trucks. If you are hauling some serious tonnage or pull a heavy trailer you might want to invest in a tire with higher load capacity. This will keep your rear tires from pan caking and improve stability.

Tread wear and Traction Finally, some numbers that are useful again! A higher treadwear rating means you'll be able to drive more miles before you need new tires again. And tires with higher treadwear ratings tend to offer gas mileage advantages because they are made with slightly harder rubber. which means less resistance. The downside to harder rubber is less cornering performance, so if it's important to you to be able to carve up some mountain roads don't go for the high treadwear rating, go with the higher dry traction rating instead.

It's time for new tires. You're a smart consumer, so you want to be sure you get the most tire for your money. The emergence of a number of quality tire manufacturers in the'90s has broadened the market on tires and kept the prices low while quality and innovation continue to improve. These days there are more tire choices than miracle weight loss programs but with a little refresher course on tires, you can trim away the fat and get yourself a set of fairly-priced, safe and durable tires for your car, truck or SUV.

 

All Those Numbers and Letters
The side of your tire may look like an ancient hieroglyphic text to you. Don't sweat it. This is your personal Rosetta Stone to help you figure out what it all means, and whether it matters to you in your decision.

Each of the markings stamped into the side of a tire have a meaning.

Some of them will be important to you, some won't. You're probably not worried about the speed rating of the tires for your Prius, but your Porsche 997 Turbo needs go-fat rubber on the road. On the flip side, treadwear ratings are important to your hybrid, since they affect gas mileage and the effect your tires have on the environment.

The Breakdown
As technology has advanced, the tire companies agreed upon a uniform system of describing every aspect of each tire. Lucky for us, they all follow the same formula (except for race car tires which are not legal for street use.) The following categories are squeezed onto the side of every tire you can buy.

 

• Vehicle Application
• Tire Width
• Aspect Ratio
• Radial Designation
• Wheel Diameter
• Load Index
• Speed Rating
• The Official DOT Stuff
• Ply Construction
• Load and Pressure Limits
• Treadwear and Traction Ratings