Car Basics 101: All About Tires

BY Scion CM
August 20,2012

In Car Basics 101: How to Change a Tire we learned necessary steps for taking a wheel off and putting it back on your Scion. There's some more information to know in order to make the best decision next time you need a new set of tires, read on for the basics.

When buying a set of tires, brand, type, size, and other ratings all need to be taken into consideration when choosing. The choices can seem overwhelming but with a little knowledge under your belt it will be easier. Usually the first question people have is what size is their tire and what do all those numbers mean?

SIZE

The size of your tire can be taken off the actual tire itself or there is also tire and load information on the door jamb of your vehicle which will tell you OEM specs. The wall of a tire has tons of information on it – for size, you're looking for something similar to P185/60R15. Let's break that down – "P" stands for passenger, the following number "185" is width in millimeters. The next number is called the aspect ratio, meaning it's the ratio of height of the tire's sidewall to width. So the smaller the number, the shorter the sidewall. "R" stands for radial and the last number is the size of your rim, measured in diameter. So basically if you just remember width/sidewall, rim size you'll be set.

If you're merely replacing tires on stock rims, generally you'll be getting the same size as your car came with. A change in your car's rims may necessitate a change in tire size. The two different wheels shown on an xB above illustrate various rim/tire size combinations. On the left is a 15" rim (OEM size) with 185/60R15 tire and on the right is a 18" rim with 225/40R18 tire.  The 18" rims have a wider tire with a shorter sidewall making its overall height about the same as stock.

TYPE

Tire type depends on the where you live and how you drive. The most common categories are touring or all-season tires. If you live in a mild weather area (or rotate in seasonal winter tires) a touring tire may be right for you. These offer a smooth, quiet ride and handle well at freeway speeds.

An all-season tire performs much better in wet conditions. It's tread pattern is designed to help push water out from under it. It's also made of a rubber compound that doesn't freeze, keeping it pliable even in very cold conditions.

Less widely used tires that could be used on a Scion would include winter tires and performance tires. For people who live in places with snow and ice on roadways, winter tires provide the best handling. Because of their composition, they shouldn't be used all year round as they'll wear out quickly riding on dry pavement.

A serious performance tire will help with grip and cornering while aggressively driving – however they're expensive, wear faster, and aren't designed to operate in weather. For this reason, racers often save these exclusively for the track.

RATINGS

Courtesy of www.safercar.gov.

As shown in the diagram, there are a number of different ratings on a tire. A load index and speed symbol may not be very important for the average driver, but is good to know what it is. The treadwear, traction, and temperature grades make up what's called the Uniform Tire Quality Grade (UTQG). This is a federally rated grading system – however each manufacturer is responsible for testing and grading their own products.

For treadwear, the higher the number the longer lasting the wheel, dependant upon proper maintenance. Traction is graded from "AA" being the highest down to "C" being the minimal grade allowed. Temperature refers to temperature resistance or the tire's ability to protect from and dissipate heat and is graded from "A" to "C".

A complete listing of these ratings, along with even more information, is available from safercar.gov/Tire.

Equipped with information, you can now make more informed decisions the next time you're shopping for some new tires. In our next Car Basics 101 article, we'll explain the most important tire maintenance steps to take in order to ensure the longest lasting, safest, and most mpg efficient wheels!