BCB Car Guide: How to Read the Tire Numbers
Tires are the only thing between you and the road so we will dive into the details on everything you need to know about tires. We will cover how to read your tire specs, when you need new tires, where to buy tires and my thoughts on plus sizing and more.
This article will focus on passenger cars since most of my visitors looking for this information are car owners. I will cover SUV and light truck tires in another article.
How to Read the Tire Numbers
Tire numbers are easy once you get the hang of what you are looking at. The first number is the width of the tire. This is measured in mm but the most important thing to know is this number since it will impact the rest of the tire's specs and performance.
In my case, my tires are listed as 255/35/R18, the 255 is the width of this tire. If you have a high-performance car or sport package, this number may range from 215mm to 305mm. The bigger the number the wider the tire.
TIP: While wider tires typically mean more grip, you will want to make sure that your tire will FIT on your car, depending on the suspension and the fenders, you will be limited on how wide you can go. We suggest that you stick with OEM specs or an equivalent size since you want the rolling diameter to be 3% or less of the factory specs.
The middle number is the aspect ratio. This is the size of the sidewall based on the tire width. Note, the lower this number the less rubber you have between you and the road. It might look great but the ride quality will suffer. I have been there and done that with 19" rims.
NOTE: If you upsize your tires and it's more than 3% from the original OEM, you run a risk of confusing the ECU (Engine computer) and will be messing up its calculations. I have seen transmission codes and errors appear with the wrong size of tires.
How Do You Know What Tire You Need?
The best approach is to look at your current car and look at the sidewall of the tire. It will be imprinted on the side wall. If you don't have your car readily available because it is parked on the street or in the parking deck, you can visit sites like TireRack.com or TireBuyer.com to input your car's information and it will display the tire options for your car.
The proper tire size will ensure best performance for your car. In fact, places like Costco will not mount the tire if it doesn't match the correct OEM tire size. This is due to the liability of having the wrong load rating and rotation for the transmissions.
What About Upgrading Your Tire Size with Larger Rims?
You can upgrade your rim size to a larger size. This is very common in the car scene. Larger wheels will look more aggressive and provide an immediate new look for your car but note that the larger the rims, the shorter or more low profile will be the side wall.
TIP: If you are in a pothole prone area or have bad roads, I recommend for passenger cars to keep your rims size 18 inches or less.
How to Find Rim Size on Tire?
The last number on the tire is the rim size. Sometimes the number might have R18 for example. This means that the tire size is for an 18 inch rim.
For example, the tire size 255/35/18 indicates that the width is 255, the sidewall is only 35% of 255mm and the rim wheel size is 18.
There are other numbers related to the rim such as offset. Typically the lower the offset the more aggressive the wheel fitments are. This means the tire and rim will stick out further from the fender. Wheel spacers effectively lower the offset of higher spec'd wheels with an offset of 48+ or 50+.
However, to keep things simple, just look at the last number and that will be your rim size.
What Type of Tire Should I Get?
There are many options when it comes to tires. While you might assume that a tire is just a tire. The brand and the technology that goes into the tire will greatly affect its performance.
Here are my simple rules to follow when replacing tires:
Weather - Depending on your location, you might opt for summer only tires vs all-season or snow tires. Each of these tires will perform differently in different temperatures and climates. Never try to use a summer tire in the snow, you will have zero grip as the tire compounds of the summer tire will be rock hard and provide zero traction.
Budget - Pricing will vary from brand and manufacturers. I have owned tier 1 tires like Michelins and tier 3 tires like Vogue Tires and it does make a difference on how the car performs on the road. Keep that in mind.
New Tires vs Used Tires - I have had good success on used tires via Ebay and Facebook Marketplace but it's always good to get the new tires and the best tires you can afford at the time. Used tires with a patch or a plug is ok but I would avoid trying to plug or patch the tire excessively.
Installation Costs - Depending on your location, cost to install and balance tires can vary. Here in the NorthEast, expect to pay between $20 - $45 to mount and balance each wheel and tire. I have had good luck with Amazon purchases with installation. With the discount it was $17.99 plus tax at my local Mavis Tire Center.
How to Read Your Tire
In addition to the tire width, type, aspect ratio and rim size there are (3) ratings that you need to pay attention to. A tire's load index tells you exactly how much weight it can support. If you want to replace your tires, make sure you pick up a set that has a higher load rating than the original ones.
Load Index
A tire load index or load speed rating will tell us how much weight our vehicles can carry. This information is important because it helps drivers determine what size tires are best suited for their needs. In addition, knowing the maximum load of your vehicle allows you to calculate the amount of cargo space you have available.
TIP: You do not want to use a lower load speed rating for your tires as you run the risk of blowouts. Be sure to use the proper load rating for your tires.
The formula for calculating the maximum load of your car is simple: multiply the individual tire load capacity times four. For example, a vehicle with six wheels and three tires each rated at 30 pounds per square inch carries a maximum load of 90 pounds.
You can look on the driver's side door for load rating and on your current tire for the number. This load rating number is after the tire width, aspect and rim size. In my case, the load rating is 95Y. This number demonstrates that it can handle more load.
Speed Rating
The letter next to the load rating is the speed rating. High performance tire would have
The speed rating system is used to determine how fast a car can go based on the size of the tires, the weight of the vehicle, and other factors. A highest Y rating indicates the highest level of performance.
The tire shop or mechanic can ensure that you will be ordering the correct size, load index and speed rating. While you will most likely never exceed the speed rating, it's good to know about the tolerances for the tire's capabilities.
Important Tire Ratings: Treadwear, Traction, and Temperature Grades
The Tire & Rim Association (TRTA), along with the Rubber Manufacturers Association (RMA), developed a grading system for tires called Treadwear, Traction, and Temperature Grades.
These three categories are used to determine a tire’s performance on different types of surfaces. Tires receive one grade for each category.
Treadwear Grade: A rating indicates how well a tire performs under wet conditions. B ratings indicate good traction and durability under dry conditions. C ratings mean the tread wears out quickly. D ratings mean there is no tread left.
Traction Grade: A rating indicates good handling characteristics. B ratings indicate average handling. C ratings indicate poor handling. D ratings indicate the vehicle cannot handle the road.
Temperature Grade: A rating indicates a tire that does not heat up excessively during use. B ratings indicate a tire that heats up moderately. C ratings indicate excessive heating. D ratings indicate extreme heating.
In addition, there is also an estimated treadlife stamped on the tire. This number will indicate how long you can expect the tires to last. The higher the numbers the longer the lifespan. However, with today's EVs and heavier chassis, the treadlife of EV cars is shortened by a year or so.
How to Know How Much Air to Put in Tires: The Right Tire Pressure
Checking tire pressures regularly is important because it helps prevent damage to the tread and sidewalls. Proper inflation ensures the best handling and safety, especially during braking and cornering. Air pressure can greatly impact the steering and the handling of your car.
Plus, having the proper tire pressure will maximize your fuel efficiency. This is so important today with rising fuel costs.
I typically recommend running air pressure of 33psi-36psi for front tires or all around. However, you can check the manufacturer's recommended pressure by opening up your driver's side door and looking for the sticker with the numbers. Please see an image for example.
If your car has Tire Pressure Monitoring Sensors, TPMS, any pressure at 24 psi or below will trigger a light. Be sure to pay attention to this.
Vehicle Owner Manual
Most manufacturers provide owners manuals online. If you don’t find yours, check the manufacturer’s website or car forums for recommended tire air pressure for best real-world performance.
Tips for Checking Tire Pressure:
It's best to check in the morning. As you drive the car, the tires will heat up and cause the pressure to rise.
Check your pressure at least once a month. It's always good practice to get into the habit of checking your pressure.
Do not over inflate your tires. It will cause rougher, stiffer ride and can potentially lead to premature tire wear in the middle of the tread.
How Often to Change Car Tires
Why do new car tires wear out so fast? Car tires typically last between 6-10 years. However, if you experience any dry rot or bubbles in the sidewall then you should look to replace them asap.
There is a DOT manufacturing number or tire date code that is required on all tires. The DOT or Department of Transportation number indicates when the tire was manufactured. You will want to have the tires be within 4-6 years of that date. Anything older, I would recommend changing.
NOTE: I have run old tires, for example my spare, when I'm in a pinch but I would definitely change tires once you hit 8-10 years from the DOT date.
You should replace tires in pairs as best practice but you can replace 1 tire if you had a blow out. In cases like this, I would look to eBay to find a used tire that would match the same tread wear of the rest of the tires. This way, they would all match and when it comes time to replace the set, you can do so for all four tires.
Tires should last between 20k to 60k miles depending on the type of driving you do and maintenance. It also depends on the even wear. Be sure that your tires are wearing out evenly. If you have inner wear or outer wear, you should look to get an alignment asap in order to prolong the life of your tires.
What about Tire Patches, Plugs and Sidewall Damage
If you have any damage like cuts or bubbles on your tires sidewall, I would highly recommend that you replace that tire. The sidewall bubble is hazardous because the sidewall is compromised and can lead to a blowout. Unfortunately, you are not able to repair the tires sidewall.
As for a flat tire, you can repair it with a patch. This is preferred because it is a stronger repair. You are allowed to patch a tire up to two times. However, my rule of thumb is to patch or repair once only.
Plugs are good in a pinch, just be aware that you should check the air pressure from time to time to check for leaks. You can pick up a DIY tire Repair Tire on amazon for around $30-$40 and keep it in your truck in case of emergencies.
Can You Mix Tires and Brand?
The tread pattern on the tires are very specific to how it performs in the rain and inclement weather. I have run a mix match tire on one corner of the car when I had a flat. It's not too critical that you have 1 tire that is different.
But I would recommend that you limit the mix match to 2 tires only. Running all different tires on all four corners can lead to unpredictable performance in wet conditions and when temperatures change. It would be best to stick to the same brand or at least a similar tread pattern. This will help keep the tire performance consistent.
Your auto tire services center or tire shop can recommend the best options for you when it comes to replacement tires.
In Conclusion
In conclusion, tire pressure is one of the most overlooked aspects of maintaining your vehicle's safety and performance. While you may assume that checking your tire pressures regularly is a simple task, it actually requires quite a bit of effort. That's because there are several factors that affect tire pressure, including temperature, road conditions, and driving style.
To help you keep track of these variables, we've created this handy guide to help you determine which tire pressures are right for you and how to care for your tires. So whether you drive a light truck, minivan, SUV, sports car, or truck, check out our recommendations below and let us know what you think!