Tires: Sizing, Pressure, Load, Speed, And Treadwear — Understand It

Tires: Sizing, Pressure, Load, Speed, And Treadwear — Understand It

Yes, in the simplest of terms, tires are round, and they are made of rubber. But trust me, tires are much more complicated. In fact, they are the most crucial component of a vehicle. Every single thing a car does, it does through the tires. Any performance part you bolt-on a project car, from a supercharger to new lowering springs, has to push that performance through the tires for any gains to be achieved. If you want to make your car better, step one: get some good tires.

For the 2019 NASA National Championships at Mid-Ohio, our team took no chances on the most important part of our car for the most important race of the season: fresh sets of Toyo Proxes RRs for qualifying and for the race.


One of the first things to know about tires is the sizing nomenclature. There are all kinds of numbers and letters on the sidewall of a tire, but being able to decipher this information will better help you make educated decisions about tire choices. Tire sizes don’t come like underwear sizes: small, medium, and large.

Tire sizes look like this, “205/40R17,” which are three separate yet equally important numbers. Some are in millimeters, some are percentages, and some are in inches. Confused? You should be. It is totally confusing.

Here is the breakdown: the first number, 205, is the lateral width of the tire in millimeters (great if you are in Europe, but confusing to us Americans who like inches). The second number (after the forward-slash) is the height of the sidewall, which is a percentage of the width of the tire, known as the aspect ratio. So a 205/40 sidewall is 40-percent of 205 millimeters. Using your iPhone calculator will tell you that the sidewall height is 205 times 0.40, which equals 82 millimeters. The last number, after the R, is the diameter of the wheel in inches.

This tire is a 205/40R17, which means it is 205 millimeters wide, with a sidewall that is 40-percent of 205 millimeters, on a rim that is 17 inches in diameter. We covered this part of tires on an earlier story on TURNology about buying tires through eBay.

If you have a car with 18-inch rims, obviously, a 17-inch tire isn’t going to fit, no matter how hard you try to stretch it on the rim. While shopping for tires, your first step is to ensure you have the correct rim diameter. In regards to which tire has the lowest profile (smallest sidewall), you have to pay close attention to both the tire width and the aspect ratio.

Because sidewall height is a percentage of the tire width, merely looking at a 45 series versus a 50 series tire won’t always give you the obvious answer. Intuition will tell you the 45 series tire is shorter; however, the math will prove that a 225/50R15 tire will have a smaller sidewall (112.5 millimeters) versus a 275/45R15 tire (123.75 millimeters) due to the wider tire (275 mm).

This is important 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 offers more grip. You have to decide what you need and what will fit your rim and within your fenderwells.

In the Honda Challenge 4 class, the rules limit the wheel width to 7 inches, and the fenders cannot be flared. The trick is to find the widest tire that will fit on the rim and still fit inside the fenders, without rubbing. We use the Toyo Proxes RR 225/50R15 and have had great success.

Load Rating

The load rating is something else to consider when buying tires, and it is often overlooked. In most cases, like in the instance of passenger cars, it isn’t an issue. Most tires have enough load rating per tire; once you put four tires on a vehicle, the combined load rating on the tires is more than the vehicle weighs.

Where the load rating can be an issue is on trailer tires or trucks. Ever wonder why people have dually trucks? It is merely the need for more load rating on the axle; four tires can handle more load than two. A standard truck with two rear tires, each with a load rating of 1,819 pounds, can only manage a total burden on that axle of 3,638 pounds (can’t put much in the bed of that truck). But, a dually truck with four 1,819 pound load-rated tires can handle 7,276 pounds.

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 pounds. Instead of looking up a load rating chart for what 97 means, just look closely at the sidewall of the tire. It will have the actual load rating listed in pounds (and in kilograms if you are from the European Union) cast into the molding of the rubber.

This Cooper Zeon RS3-G1 245/40R18 tire has a load rating of 1,609 pounds per tire. That means with two tires on an axle (one on the left, one on the right) each axle can weigh up to 3,218 pounds and the entire vehicle (including fluids, cargo, and passengers) can weigh up to 6,436-pounds total.

The simple thing to remember about load rating is that the vehicle (and it’s cargo) can’t weigh more than the sum of the load rating of all four tires. If you have that covered, you have nothing to worry about.


Tire pressure is immensely important in the performance and safety of a tire. Having an accurate and reliable tire gauge can make a difference between winning and losing. We use a digital tire gauge that measures to the tenth of a pound.

How much air is inside the tire makes an incredible difference in how a tire performs on the track as well as driving down the street to pick up milk at the store. Setting the tire pressures when cold and understanding that when the tire warms up, the air inside of the tire will expand (increasing tire pressure) can help you set cold pressures for hot performance. We covered this in an earlier article about how to use a pyrometer to set tire pressures on a racecar accurately: Just The Tip: How To Use A Tire Pyrometer To Go Faster.

For tire pressure, the best place to find what your tire pressure should be is from the sticker located on the B-Pillar of your open driver’s door. There, you will find both the OEM tire size and the recommended pressures.

The correct tire pressure can honestly be a life and death situation. According to retired Sergeant Bill Esmay of the California Highway Patrol, who spent years reconstructing accidents, the correct tire pressure can keep a tire from coming apart. “Tires are created during a process called vulcanization — essentially, they are built with heat,” said Sergeant Esmay. “Since tires are created with heat, they can also fall apart with heat. The low pressure and overloading of a tire can cause a lot of heat in a tire that ultimately can allow the tire to come apart, which could be disastrous.” You heard it here first kids: check your tire pressures.

Old age, lack of use, low tire pressure, and possible overloading of this tire were the culminating cause of the tire coming apart on our race trailer. Luckily, we had a spare and lots of tools in the trailer. We were able to fix it and make it to our race event.

D.O.T. Legal

For a tire be sold and used on passenger cars on public roads, they must be Department of Transportation (DOT) legal. Many racing sanctioning bodies also require tires to be DOT legal. When you look at DOT-legal racing tires, you will find they are never complete race slicks. There are always some grooves in the tire.

In the case of the Toyo Proxes RR, you will find exactly two grooves in the tire. These grooves are what make the tire DOT legal. It comes from the language in the vehicle code that specifies legislation on tread depth. California Vehicle Code section 27465(b)(1) states, “On a highway, no person shall use a pneumatic tire on a vehicle axle when the tire has less than the following tread depth: one thirty-second of an inch tread depth in any two adjacent grooves at any location of the tire.”

When you read the law, it becomes obvious why DOT race tires have just two adjacent grooves: to satisfy the requirement on tread depth while also providing as much surface area between the tire and the roadway as possible. Essentially, it is a slick tire with just enough (two grooves) to make it DOT legal. Pretty smart.

DOT tires also come with a DOT date-code, a stamped code placed on the tire indicating the week and month a tire is manufactured. For instance, a tire branded with 2317 means the tire was built during the 23rd week of 2017. A tire will only have the DOT date-code stamped 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.

On the left is the Toyo Proxes RR, the sticky and fast spec tire for both NASA Honda Challenge and NASA Spec Miata. The RR has two grooves to be DOT legal. On the right, is a DOT date-code, 4518, indicating this tire was manufactured during the 45th week of 2018 (which was November of 2018).

Armed with the information regarding how old a tire is based on the DOT date-code, we asked Cooper Tires how long a tire should be left on a vehicle. According to Mohit Jain, vice president of commercial operations for Cooper Tires, “Tires ten or more years old should be replaced, even if the tires appear to be undamaged and have not reached their treadwear limits. Most tires will need replacement before ten years due to service conditions. This may be necessary even if the tire has not yet reached its treadwear limits.”

Yokohama seconded Cooper Tire’s recommendations on tire life. “With proper care and storage, a Yokohama tire can be on a vehicle up to ten years,” said Bob Abram, Yokohama’s senior manager of consumer product planning.

Speed Rating

Tires also come with a speed rating, essentially a design standard for the highest speed the tire should be used. For a long time, Z-rated tires were the fastest rating you could find on the market. But, the standard for a Z-rated tire was just “a maximum speed capability of above 149 miles per hour.”

The problem was the rating didn’t indicate how much above 149 miles per hour the tire was safe. Later, ratings for W (168 miles per hour) and Y (186 miles per hour) were introduced. The vital thing to do is to ensure your speed rating is correct for the use of the tires. If you are heading to the Autobahn, you don’t want to have tires that are only rated for 112 miles per hour.

At the Mojave Magnum 1.5-mile high-speed run, we took a 2006 Z06 Corvette and went 180.8 miles per hour through the speed traps. If we hadn’t shown up with Y-speed-rated tires, the tech inspectors wouldn’t have let us run the event. The different speed-rating letters, and their associated speed in miles per hour, are charted above.


We have talked about sizes, pressure, load rating, speed rating, and DOT legality, but what we haven’t mentioned (and it is playing a significant role in racing right now) is treadwear. If you look at the SCCA Solo rule book, section 13.3 TIRES (A) Specifications (1.), it states, “Minimum UTGQ Treadwear Grade of 200” is required in the Street class.

UTGQ stands for Uniform Tire Quality Grading, a DOT standard for measuring tire wear. According to Cooper Tire’s Mohit Jain, “The treadwear grade is a comparative rating based on the wear rate of the tire when tested under controlled conditions on a specified government test track. As an example, using 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. So, this tire wore three times longer than the control tire.”

To compete in the SCCA Solo Street class, not only do you have to use a DOT tire, but it also needs to have a 200 treadwear rating. The FordNXT project car, a 2019 Ford Fiesta ST (The Way of the Fist), is currently rocking the Yokohama Advan A052 200 treadwear autocross tire.

Lower treadwear numbers are undoubtedly linked to a stickier tire that wears quickly. For instance, a Toyo Proxes RR has a treadwear rating of just 40 (which makes it very fast around a racetrack). To keep race car speeds lower on the track, sanctioning bodies like The 24 Hours of LeMons and ChampCar (formerly ChumpCar) require the use of a 200 treadwear tire (to keep DOT race tires like the Toyo RR off their race cars). This 200 treadwear number is the same treadwear SCCA Solo uses for its popular Street classes, which makes the marketplace very competitive at this treadwear number.

There is a tire war occurring between manufacturers based on SCCA rules for Street class autocross requiring a 200 treadwear tire. Racing sanctioning bodies like the 24 Hours of LeMons and ChampCar are also requiring a 200 treadwear tire, which is increasing the marketplace for high-performance tires with a 200 treadwear rating.

Yokohama recently introduced its newest 200 treadwear tire, the Yokohama ADVAN A052, which according to Yokohama, is an adaptation of their AD08R tire. “The original ADVAN Neova AD08R started as a 180 treadwear when it debuted in 2013,” said Duane Sampson, Yokohama product planning manager.

“The 200 treadwear rule for SCCA became effective on January 1, 2015, so we were behind, and had to adapt that tire to become a 200 treadwear. The ADVAN A052 was conceived and planned to be a 200 UTQG tire from the outset. We built it with the focus of competing specifically in those series that use 200 treadwear as a rule. So, the inspiration was not just to compete, but to win!”

The tread layout of the Yokohama ADVAN A052 is designed for dry-surface performance and was built for one reason, and one reason only — to win any 200 treadwear class series.


When it is the off-season, race cars often get put away and left alone for a few months, which is not suitable for a racing tire. We spoke to Marc Sanzenbacher, senior manager of motorsports at Toyo Tire U.S.A., and asked him the best way to store racing tires. Marc had a lot to say on this issue.

“The best practices for race tire storage are as follows,” he says.” Remove the race tires from the vehicle and deflate them to half the normal air pressure. With frigid temperatures at night and warmer temperatures during the day, this reduction in vehicle load and air pressure allows the tire to better handle the more extreme expansion and contraction of the air in the fall and winter. Store your race tires as clean as possible but not oiled up. Wipe them off, or better yet, wash them with simple soap and water to get any oil, brake dust, or dirt off them. Protect the tires from exposure to direct sunlight and ozone.

“Both UV light and ozone can deteriorate the compounds used in race tires. When not in use, race tires should be stored in protective tire bags, away from machinery such as electric generators and motors that can emit ozone. Store them in a cool, dark place above freezing temperatures, and keep them in an environment of 68-degrees Fahrenheit or higher for at least 24 hours before mounting or dismounting. If you are like some and must remove them from the rims, then the tires should be stacked on their sides rather than upright. But, don’t stack them more than four high.”

For our Honda Challenge Toyo Proxes RR tires, during the offseason, we store them inside to keep them away from cold temperatures. However, we did stack them up to as much as nine-high in the breakroom, which is a mistake. According to Marc Sanzenbacher from Toyo, we should only be stacking these tires four-high while they are in storage (we will fix it).

Show Me The Money

Not only do Toyo and Yokohama build extremely high-performing tires for racing, but they also support racers with contingency programs. NASA racers who use Toyo Tires and run Toyo Tire stickers on all four sides of their race cars are eligible to win Toyo Bucks: money to purchase more Toyo tires. There are similar tire contingency programs for SCCA racers.

Running the right tire can put money in your pocket. Winning the NASA National Championship and the SoCal Regional Championship put thousands of dollars in my team’s coffers thanks to Toyo Tires and its contingency program. You can see in the photo above, I have my Toyo Tires stickers in the appropriate spots to satisfy contingency requirements.

Yes, tires are round, and they are made of rubber, but you have just learned there is a lot more to them than that. Tires can save your life, and they can win races. Ensure you have the right donuts on your ride, they are correctly maintained, and at the correct pressure. Then, go out and win some races!

Article Sources

About the author

Rob Krider

Rob Krider will race absolutely anything. He is a multi-national champion racing driver and is also the author of the novel, Cadet Blues.
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