Tires 101: Understanding Sizes & Options
When you think of car safety and your children, what comes to mind? For us moms, it’s probably car seats. But what about something as seemingly basic as tires? After all, they are your vehicle’s point of contact with the road, and every move you make in the car—from braking to steering to accelerating—is transmitted to the road by your tires. Outfitting your vehicle with a set of quality tires can make the difference when it comes to keeping your family safe on the road. However, choosing and buying tires can sometimes be intimidating if you don’t know what to look for. Expensive tires do not always translate to better tires for your particular vehicle and lifestyle. Below we offer a guide to understanding the characteristics and sizing of tires, the ‘why‘ behind each, and some information to help you make an informed decision when it comes to choosing the right tires for your car and your family’s needs.
Beyond tread, do you know the other parts of a tire? Most tires today are made from upwards of 25 different components! Underneath the tread of a tire are steel belts that keep the tire stable and keep the tread flat so the most surface area possible has contact with the road. Within the core of a tire are several more layers of fabric, rubber, and other materials referred to as “plies.” The layers of plies have cording which can be arranged in different patterns, the most common being a radial design. The sidewall of a tire is the portion where the tire’s dimensions, ratings, and additional information can be found. The characteristics of the sidewall determine a lot about the tire’s performance. A tall, soft sidewall typically makes for a smooth ride; while a shorter, more rigid one may make sharp turns easier.
How To Read Tire Sizes
In order to shop for tires you of course need to know how to read sizes and know the requirements of your particular car. Check out the graphics below to learn how to decipher sizes and then read on for how all of these specifications combine to create the “personality” of a tire.
Tire size and ratings are stamped on a tire’s sidewall along with some additional information discussed below.
You can usually find the specifications of the original tires that came with your car listed on the placard on your driver’s side door frame or on the inside of the fuel door. In addition to listing proper tire inflation for your vehicle, this label includes the size of what is referred to as your car’s original equipment or “OE” tires. When determining what to look for in a set of new tires for your car, it’s best to consult this placard rather than the sidewall of your current tires, in the event that they are inappropriate for your car (unlikely, but best to be safe). Car makers put a lot of research and development into selecting OE in order to complement the particular features of your vehicle. If you like the way your car rides, then you can simply look for tires that match your OE. However, if you’re not satisfied with the way your original tires handle the road or if your needs change, you can certainly upgrade from the OE. There are some specs that you want to make sure match though, such as load index.
Putting It All Together
As you can see from above, tires are not made up of one size but a variety of characteristics that come together. Here’s how everything relates and what it means when you’re on the road.
- Tire Class – The first character printed on a sidewall is usually P for a passenger car or LT for light truck. LT tires usually can carry a heavier load and are found on pickup trucks and SUVs, although the original SUV tires can also be P class.
- Width – This is in millimeters. When you make a tire wider, it generally gives it a better grip since more surface area contacts the ground, making braking for emergencies easier. However, more grip means your engine has to work harder to move the vehicle and you lose efficiency. Changing the tire size may change how evenly your tires wear as well.
- Aspect Ratio – The aspect ratio (also sometimes called the tire’s series or profile) is a percentage based on the tire’s height divided by its width (higher aspect ratio=taller tire). As a benchmark, all-season passenger car tires are around 55-70. In general, smaller aspect ratios (shorter tires) make for easier sharp turns (or “cornering”), but the smoothness of the ride will be affected. Inversely, a taller tire provides more cushioning to your vehicle and makes for a smooth ride, but the trade-off is reduced steering response.
- Tire Construction (aka “Ply Rating”) or sometimes Load Range – This relates to the plies. You shouldn’t worry much about this designation as radial (“R”) is pretty standard (unless you own classic cars or agricultural equipment!). In lieu of an “R”, there may be no designation. You may also see a load range such as SL (standard load), LL (light load), or XL (extra load) following the rim diameter (see below). In general, it’s best to stick with a load rating that matches that on your original tires. If you think you may want to upgrade or downgrade, discuss with your tire dealer in order to determine your specific needs.
- Rim Diameter – The rim diameter designation gives the size of the rims the tires must use. This must match the rims in order for the tires to be mounted properly and safely. When selecting new tires, you also want to be sure your new tires have an accurate overall wheel diameter. This overall diameter is determined by the width, aspect ratio, and rim diameter all together. Your tire dealer can tell you the overall diameter, or you can usually find it in the tire manufacturer’s product information. For cars and vans, staying within +/-3% of the OE width is desirable, while SUVs and trucks can usually handle up to a 15% wider width (1). An accurate diameter is important in order for speed data processed by your car’s computer to be interpreted correctly.
- Load Index – This number is a code that represents the load the tire is capable of carrying when inflated to its maximum safe pressure. To get an actual load expressed in pounds, you can look up the index number in a chart which will tell you an actual value in pounds (per tire). While some family SUV’s may come with P-class tires, you may want to consider switching to a tire with a higher load index if you plan on packing up the car for family vacations (let’s face it, we all know many family trips involve packing everything but the kitchen sink!) or pulling a trailer.
- Speed Rating – The speed rating is all about how well a tire can dissipate heat at higher speeds. The more heat, the faster your tire will wear and break down. In general, higher mph ratings may be associated with tires designed to provide better traction and handling. Your best bet to ensure your tires are matched to your vehicles specific capabilities is to stay with speed ratings equal to or higher than designated on the placard in your car.
Additional information you’ll find on your sidewalls
UTQG rating – This Uniform Tire Quality Grading is the Department of Transportation’s label for a tire’s tread wear, temperature resistance and traction. Since testing can vary between brands, it’s really only useful to compare these ratings among tires of the same brand.
Tread Wear – A tire with a tread wear rating of 300 should last three times as long as one with 100, assuming they’re subject to the same conditions. Summer tires will usually have lower ratings than their all-season counterparts. Winter tires usually aren’t even rated as they wear more quickly if driven in dry conditions.
Traction – Traction is also rated AA to C for traction specifically on a wet road, with AA offering the best grip on a rainy day.
Temperature Rating – This is another rating that’s tied to speed capabilities (like the speed rating indicated in the tire size). There are only three ratings, A to C, and you should match at least to your original tires, though if you match the speed rating then this rating becomes somewhat redundant.
DOT Compliance Code – Not a very interesting number, except for the last four digits which tell you when a tire was made, the first two numbers being the week and the last two numbers being the year.
M+S Markings (or lack-of) – All-season tires will carry a label of M+S for mud and snow, while summer (or “three-season”) tires will have no markings. Winter tires will usually be marked with an M+S plus a mountain/snowflake icon.
Maximum Inflation Pressure – You may also see this value listing on the tire, however this is not necessarily the ideal pressure, but simply the maximum inflation pressure without taking into account the particular vehicle it’s mounted on. Instead, always go by pressure guidelines on the placard of your car.
Keep in mind that above and beyond these specs, different materials, compounds of rubber, tread design, and much more affect the ultimate performance of a tire. So, comparing values such as speed rating or aspect ratio really only makes sense when you’re looking at different sizes of the same tire (this is especially true for comparing tread wear ratings).
Determining Your Needs
Most owners of family cars or pickup trucks will only need to choose between three types of tires: all-season (aka touring) tires, summer (aka three-season) tires, or winter tires. A good strategy to help you decide is to consider the worse conditions you’ll encounter. Live in snowy Syracuse where you’ll need to venture out in storms? Get winter tires put on just for the season. Rainy Pensacola, Florida? A summer tire with a water-wicking tread pattern is your best bet (yes, even better than an “all-season”). And if you live somewhere that sees a little bit of every season, but nothing extreme, then all-season tires provide the best compromise if you want to keep the same set of tires on your car year-round.
What Does Driving A Good Tire Feel Like?
Daily Mom recently joined a group of moms at a Cooper Tire test track facility to experience first-hand the difference between tires. We put the tires through their paces in real-life conditions on a wet pad with lots of cornering, braking, and even spinning out. Each mom got to test both Pirelli tires (a highly respectable tire that’s often used as the OE on sports cars!) and Cooper Tire’s latest touring tire, the CS5.
The results? All 15 moms unanimously agreed that the CS5 performed way better by a long-shot. In fact, none of the moms could even get the CS5 to spin out on the wet surface. If you’ve never had the chance to test tires under controlled circumstances like this then it’s hard to appreciate what a difference a change in tires can do. Here’s what we moms perceived:
A good set of tires feels responsive. When you’re making a sharp turn or making an emergency detour, you’ll feel your car and tires respond to the movements of your wheel—you don’t need to over steer; good tires will “talk back to you.” The right tires will make you feel as if you have a direct line of communication to the road. In wet conditions, you’ll feel as if your car has more grip with every movement (and the road may “not even feel wet!”).
A Look at the CS5 by Cooper Tire
Again, it is not only the specs and tire size that translate into a safe, smooth ride, but additional technologies, materials, and tread patterns that form a tire’s personality. Below is a look at Cooper Tire’s newest design to give you an idea of additional features to look for. Many of the design characteristics in the CS5 make it a tire with a great “personality” for moms whose top priority is keeping her family safe on the road but want to stay within budget.
- Wear Square — This feature is perhaps the one most drivers will appreciate. A raised square of rubber directly on the tire’s tread gives a visual indicator of how worn your tread is as well as well as providing a warning of misalignment (through uneven wearing of the square).
- Silica Compounds — A great deal more silica is used in the CS5 which translates into better grip on wet roads without sacrificing fuel economy.
- Siping — Siping are thin slits in the tread, creating tiny “rivers” for water to be evacuated. A 3D and interlocking siping design on the CS5 further improves performance in wet conditions.
- Asymmetric Tread Design — The tread pattern on some tires limits them to use on certain positions, but an asymmetric tread design on the CS5 means the tires can be rotated to different positions on the car which means they’ll last your family longer.
We think all moms should arm themselves with an understanding of tires, sizes, and their choices when it comes to buying. Whether you want to research your options online ahead of time, or just be able to pop into your local tire dealer and have a conversation about what you need, knowing your car and tires is the first step in keeping your family safe on the road.
Tags: auto, auto terms, buying tires, car, car lingo, car parts, car talk, Cooper Tire, CS5, educate yourself, how to choose tires, researching tires, shopping for tires, shopping for your car, Supermom Ride-N-Drive, tips for moms, tire basics, tire features, tire guide, tire safety, tire sizes, tire sizing, tire terms, understanding tires
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Laura is a wife and a new mom living on the East Coast. She and her husband welcomed their first child in September 2013. She has a passion for photography and an incurable case of Wanderlust. In her spare time she enjoys blogging about photography, travel, married life, and motherhood at The Whimsical Photographer.