Tire Speed Rating, Tire Sizes and Tire Aspect Ratio
If you own an automobile, you are eventually faced with the situation of replacing the tires on the it. But this can be a daunting task for the inexperienced. How do you determine the type of tires your car needs and what do all of those numbers on the side mean?
In this article I would like to provide some basic information as a resource for anyone that has ever been confused by all of those numbers so that you can understand what you are buying, and not get upsold on something you do not need.
Difference Between Wheel and Tire
The first distinction you need to understand is the difference in terminology. A wheel is not the same thing as a tire. If you have a bad blow out on the road and go to a shop and tell them you need a new wheel, you are going to be shown something different than what you actually need.
The wheel of a car is the part that the actual rubber tire goes on. Think of the wheels as the rims on your car if that helps in understanding.
However, the type of wheel on your automobile will determine the type of tire you can put on your car so it is important to know the size of wheel, which is measured by the diameter of it across the center.
Most passenger sedans have 15 or 16 inch wheels. For instance, my mother’s Honda Accord has 15 inch wheels, but a larger sedan might have 16 or even 17 inch wheels. My Nissan Maxima comes with 17 inch wheels.
Most economy cars have 14 or 15 inch wheels, although I have even seen them with 13 inch wheels. Economy cars use smaller wheels and thinner tires because it helps with fuel economy.
Now that you have a basic understanding of wheel sizes, you can use that to determine the correct tire size.
Stick With Car Manufacturer Tire Recommendations
When you buy a new car, the tires that are on it have been selected because they provide the optimal fuel mileage, road noise reduction and comfort for that model. This decision is arrived at after extensive testing by the manufacturer.
In most cases, you should go with the car manufacturer’s recommendation else you could throw off various settings such as the accuracy of your speedometer, fuel mileage, braking distance and cabin noise.
However, if you want a more aggressive looking tire, you can go up a size in width or diameter usually without issue, although you might want to get your speedometer recalibrated.
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What do Tire Ratings Mean?
A typical size and rating looks something like this. It is displayed prominently on the sidewall.
Example: P215/65R15 89H
What do those numbers and letters mean?
Service Type of Tire
The "P" refers to the type of car it should be used for, in this case it stands for passenger. Other designations include:
- T = "Temporary Spare"
- LT = "Light Truck" size designed for vehicles used for towing or carrying heavy loads.
- C = "Commercial" used on delivery vans and trucks
- ST = "Special Trailer" sizes should never be used on cars because they are specifically designed for boat or utility trailers.
"215" represents the width in millimeters. The higher the number, the wider the tire, so in this case, it will make more contact with the road than say a “190” size tire. More contact with the road also means less fuel economy.
However, if you want a more aggressive looking car, the wider the better.
Sidewall Aspect Ratio
Referring back to the example above, the "65"after the slash is the ratio of height to width. It represents the tire's profile, or aspect ratio. The higher the number, the larger the sidewall, or the area from the rim (or wheel) to the tread on the outside.
If you have ever seen those silly looking tricked out cars with sidewalls that are only a couple of inches, that means this number would be much lower than 65.
If you are considering a low profile look, understand they do not get very good fuel mileage and are prone to damage on the sidewall that is impossible to repair. They also have a reduced lifetime mileage expectation.
Internal Construction Rating
The letter after the aspect ratio indicates the internal construction of the tire. The most common type of passenger tire will be rated R which stands for Radial construction.
This is a description of how it was developed, meaning that the layers or materials used in making it radiate out from the center of the wheel.
There are other less common ratings for internal construction, such as D which is used for light trucks and spare tires.
If you own older ones, you may see a B rating which signifies belted tires, but those have been
There is also an F designation which identifies so called “run-flat” tires. It will usually follow the normal rating for a radial so using the example above, the size would appear as P215/65RF15 89H
Given that virtually all passenger vehicles are sold with radial tires, R rated tires are the only ones you should concern yourself with.
Tire Speed Rating
Car tires used to have a speed rating on the sidewall, however since 1991, they are only rated for speed in this visible manner are sizes identified with a “Z”, such as P215/65ZR15 89H.
All of them still carry speed ratings of course, but it is located in the service description which is indicated in the last set of numbers and letter.
Referring back to the original example, the 15 indicates the diameter, in this case 15 inch. That means that the wheels on the vehicle are 15 inch rims. You cannot put 16 inch tires on 15 inch wheels. If you want to go up a size, you have to purchase new wheels too. The tire and wheel diameters must be equal.
The last indicator in the example is called the service description.
This number and letter indicate the load index of the tire and the speed rating discussed earlier. This is critical information as it tells you the maximum safe load the tires can carry, as well as the maximum safe speed.
In our example, the 89 indicates the tire can safely carry 1279 lbs. The higher the load index number, the greater its load carrying capacity as shown in the chart.
The letter in the service description represents the speed rating H. This letter code indicates the maximum safe speed the tire can handle.
Finally, a rating of M or S indicates it is rated for mud or snow respectively.
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Other Designations on the Sidewall of Tires
Other ratings on tires include “DOT", which certifies it complies with all safety standards
Tires also contain an 11 digits serial number made up of letters and numbers.
The DOT regulations require passenger car tires to be rated on three other factors.
The treadwear rating is a grade assigned by after the tire is tested on an approved government test track. However, it should not be an indicator of how long the tread on the tire will last, or how many miles you can expect to get out of the tires. Wear depends on a number of factors beyond your control, and these test grades are based on ideal, controlled conditions.
Furthermore, the grade is based on a comparative rating in the way the Richter scale measures earthquakes, in magnitude. For instance, a quake measuring 6.0 on the Richter scale is 10 times as strong as a quake measuring 5.0
Treadwear is measured in a similar way. A treadwear rating of 200 does not have 100 times the tread life, it is simply twice as long as a tire with a grade of 100. Keep that in mind when comparing tires and do not get lost in the numbers.
Traction is graded by letters (AA, A, B and C) with AA being the highest and C being the lowest. Traction grades give an indication how well tires can stop on wet pavement, again under controlled conditions at a government test facility.
Tire Temperature Rating
Temperature grades are also measured by A, B or C, which high being the highest and C being the lowest resistance to heat generated under laboratory testing.
One other indication you should pay attention to on your sidewall is the maximum or recommended tire pressure. You should never put more than this amount of air into your tires, so be sure to monitor the pressure gauge when inflating your tires.
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