When it comes to tires, we all know they’re round and made of rubber – but trust me, there’s a lot more to tires than them just being inedible rubber donuts. The fact is, tires are the most important component on a vehicle because every single thing a vehicle does, it has to do through the tires.

Any go-fast goodies you bolt on a performance car, from a lightweight exhaust to new lowering springs, has to push that performance through the tires for any gains to be achieved. It doesn’t matter if its SCCA Autocross, RallyCross, Time Attack, RoadRally, or Road Racing, if you want to make your car better, step one is: good tires.

Because tires are so important, there’s a lot to know about them. From sizing to load rating, pressure, DOT stamps, and treadwear ratings, there’s a lot to consider when it comes to tires – and these five basics will get you on your way.

Sizing

One of the first things to know about tires is sizing nomenclature. There are all kinds of numbers and letters on the sidewall of a tire, so being able to decipher what this information is will help you make educated decisions about tire choices.

Tire sizes don’t come like underwear sizes: small, medium, and large. Tire sizes look like “205/40R17,” which is three separate equally important numbers. Some numbers are in millimeters, some numbers are percentages, and some numbers are in inches. Confused? You should be! It is totally confusing.

Here’s the breakdown:

• The first number, 205, is the lateral width of the tire in millimeters (great if you’re in Europe, but confusing to us Americans).
• The second number after the forward slash is the height of the sidewall – this is a percentage of the width of the tire. So, a 205/40 sidewall is 40% of 205 mm. Your phone calculator will tell you that the sidewall height is 205 x 0.40, which equals 82 mm.
• The last number, after the “R,” is the diameter of the wheel in inches. Which means if you thought the R stood for radius, you would be wrong. (If you want to know, the “R” actually tells you the construction type of the tire, with most modern tires being “radial.”)

I told you it’s confusing.

(This code isn't a code at all, it's telling you the exact sizing information you need to know.)

While shopping for tires, your first step is to ensure you have the correct wheel diameter. In regard to which tire has the lowest profile (shortest sidewall), remember that sidewall height is a percentage of the width, so simply looking at a 40-series versus a 50-series tire won’t always give you the obvious answer since a 225/50R15 tire will have a shorter sidewall (112.5 mm) than a 275/45R15 tire (123.75 mm). Why this is important is because your overall tire diameter ultimately changes the final drive of the vehicle. A shorter tire provides a lower gear ratio than a taller tire. A wider tire generally provides more grip. You have to decide what you want for performance and what will fit your rim and within your fender wells.

Load Rating

Load rating is often overlooked, especially when shopping for performance tires. That’s because in the case of most passenger cars, it simply isn’t an issue.

Most tires have enough load carrying capacity per tire for street cars, so once you put four tires on a car, the combined load rating of the tires is more than the vehicle and its passengers will weigh. Where the load rating can be an issue is on trailer tires or trucks.

(After this big problem on the way to an SCCA Autocross in Las Vegas, I examined the load rating on this trailer tire later weighed my overfilled trailer and realized I had packed the trailer with too much stuff for the rating on the tires...which predictably resulted in a tire failure.)

Ever wonder why people have dually trucks? One of the reasons is to increase the load rating – four tires can handle more weight than two. A standard truck with two rear tires (where the tires have a load rating of 1,819 lbs) have enough tire to handle a load on that axle of 3,638 lbs. A dually truck with four 1,819 lb load-rated tires has a theoretical tire weight-carrying max of 7,276 lbs.

There are shortcut codes on the tire for load rating, like the number “97” on a sidewall will tell you the tire is rated for 1,609 lbs. Instead of looking up a load rating chart for what “97” means, just look closer at the sidewall of the tire and it will tell you the actual load rating in pounds.

To put it another way: When it comes to performance cars, while “load rating” is written on the tires, it’s largely not a concern. When it comes to trucks and trailers, this number is vitally important.

Pressure

How much air is inside the tire makes an incredible difference on how a tire performs on the track, and also while just driving down the street to pick up some Cap’n Crunch Berries.

The reality is, the right tire pressure can become a life-and-death situation, as the correct tire pressure can keep a tire from falling apart. Tires are created during a process called vulcanization; essentially, they are built with heat, and since tires are created with heat, they can also fall apart with heat. Low tire pressure and the overloading of a tire (see “load rating” above) can cause a lot of heat in a tire (from too much flex) that ultimately can allow the tire to come apart – which can be disastrous. You heard it here first kids: Check your tire pressures.

Aside from keeping tire pressures in the optimal spot for everyday driving (especially those trailer tires), in motorsport applications, tire pressure can make an enormous difference in how the tire interacts with the roadway. Too much pressure will bulge the center of the tire, creating a smaller contact patch with the roadway and making the car skittish; too little tire pressure will allow the sidewall to deflect and de-bead the tire from the rim. Not good.

(For tire pressure, the best place to find out what your car’s tire pressure should be is from the sticker located on the B-pillar of your open driver’s door. There, you will find the OEM tire size and recommended pressures. This is a great starting point for street use, and even some motorsports events.)

Adjusting tire pressures can help you optimize the performance and safety of your car. A good starting point for the right tire pressure for your particular car is by looking at the sticker located on the B-pillar of your open driver’s door. While this number is good, what’s even better is asking for advice from someone at your next SCCA event who drives a similar car. Club members are a friendly bunch, so chances are, they’ll point you in the right direction.

DOT Code

Believe it or not, tires have a “born-on” date on the sidewall. The date isn’t obvious – you won’t see a marking that says, “This tire was created on July 17, 2024.” Instead, you’ll find on the tire the marking “DOT,” then a set of numbers. It’s the last four numbers that mean something.

The first two numbers of those four are the week of the year the tire was built (from 01 to 52) and the last two numbers are the year (ie: “24” for 2024). For instance, a tire stamped with “2924” means the tire was built during the 29th week of 2024 (or mid-July 2024).

A tire will only have the DOT date code stamped or molded on one side. If you don’t see it on the outside sidewall, chances are you will have to climb under the car with a flashlight to find it on the inner sidewall.

(Tires tell you a lot. In this instance, the last four digits in this series of numbers tells us the tire was manufactured during the 48th week of 2021.)

Who cares how old a tire is? Well, it matters for a couple of reasons. There’s no law mandating this, but the general consensus is tires should be replaced if they’re anywhere from six to 10 years old, even if the tire appears to be undamaged and hasn’t reached its tread wear limits. Many manufactures suggest not using a tire that is over six years old. European laws indicate stores cannot sell you a tire that is over five years old.

The second reason the “born-on” date matters is because over time, tires get harder; for racers, we generally want the softest tires possible. When I’m at a tire shop, I sift through the stack and get the newest DOT numbers I can find.

UTQG

Since racers like soft, sticky tires, then the treadwear number – known by tire engineer nerds as the UTQG (Uniform Tire Quality Grade) – is something many competitors look at.

The treadwear grade is a comparative rating based on the wear rate of the tire when tested under controlled conditions on a specified test track. For example, a tire with a UTQG of 300 represents the tire's comparative wear when measured against the government-mandated tire, which is rated at 100. Essentially, the 300 treadwear tire wore three times longer than the control tire.

The treadwear number is important because some motorsport classes mandate a specific treadwear number. If you look at the SCCA Solo Rules section for the Street Class, section 13.3 Tires (A) Specifications (1.) states “Minimum UTQG Treadwear Grade of 200.” That means a tire’s UTQG must be 200 or numerically higher (in a 200 UTQG class, you can compete with a 400 UTQG tire; you cannot compete with a 100 UTQG tire).

Lower treadwear numbers can certainly be linked to a stickier tire that wears quickly. For instance, a Hoosier A7 has a treadwear rating of just 40 (which makes it very fast around a road course or autocross). The treadwear number on the tires of your mom’s SUV? Probably 800.

(There is a bit of a tire war currently occurring between manufacturers based on SCCA rules for Street category autocross requiring a 200 treadwear tire, which is increasing the marketplace for great performance tires with a 200 UTQG rating.)

Obviously, there’s more than an entire universe of knowledge to know about tires, but these five things will at least help you understand how to read the tire’s sidewall. This will help for sizing and load rating, how to decode the “born-on” date, and what “UTQG” actually means.

Now, you can be better informed to spend your weekend surfing TireRack.com to find the newest tires to make you go fast in 2025.

(About the author: Rob Krider is a national champion racer, the author of the novel Cadet Blues, and is the host of the Stories and Cocktails podcast.)

Main photo by Adam Brooks. Inset photos by Rob Krider