The sidewalls of your tires have a lot to say through the markings on them. Many car owners are confused about all these tire indications, but it doesn't take them long to decode their meaning. So don't worry, because you are not alone. And if you felt confused about the tires, today we will put them all in their place.
In this article, we are talking specifically about the speed ratings and load-carrying capacity of your tires. We must consider these indications whenever you choose a set of new tires. If we do not respect the standards suggested by the tire manufacturers through tire inscriptions, we risk adding stress to them, which leads to the destabilization of the vehicle on the route. And we don't want that, do we?
Well, the indications regarding the speed rating and carrying capacity can be found right on the tire sidewall. There you will find a sequence indicating tire size and other markings. Also, you will find the speed and load ratings immediately after the tire size description and it is represented by a two or three-digit number next to a letter. Let's see how to decipher this code more precisely.
Tire Speed Rating Exemplified
Everything is much simpler when speaking in examples, isn't it? Let's look in more detail at the following tire instructions as an example: 225/40 R18 94Y. This is how most of the main inscriptions on the tires look. This data contains the most essential information about your car's tires.
What interests us today is precisely the 94Y indicator. It tells us about tire service description, i.e. about load and speed ratings. The first number, i.e. "94", tells us about load carrying capacity, otherwise known as load index rating. The following letter is a coded indicator that refers to vehicle speed rating. In our case, the speed rating is "Y" and we will soon talk about what this means exactly. 1
These 2 clues are all you need to know about the tire when it comes to carrying capacity and its speed rating. There's nothing difficult about it. All you need is to know how much this means quantitatively for your tire and how much you can push with your vehicle based on these indications. So, let's go on.
What is Tire Load Rating?
Now that you know where to look for it and how to decipher it, let's see what this index actually means. This indication informs the driver of the maximum capacity that the car's tires can carry. This does not mean that if the tire says "94" then it can carry 94 kg. "94" actually refers to an index that equates to a certain weight. So it is an abbreviated form of indicating the maximum weight allowed for a tire.
The load indices can vary from 70 to 126. The higher the index, the more weight the tire can bear. There is an international standard that all tire manufacturers use. You can see this standard in the chart below, which indicates the load rating of the tire at maximum air pressure.
According to this chart, we see that the "94" index corresponds to a load of 670 kg or 1,477 lbs per tire. And that would mean that the load on all 4 tires can be 5,908 lbs maximum.
What Load Rating Do You Need?
These indications suggest THE MAXIMAL carrying capacity index, because it is based on THE MAXIMAL allowed pressure in tires. But don't forget that the manufacturers recommend driving the vehicle with THE RECOMMENDED pressure, which is usually a little less than the MAXIMUM allowed. Respectively, the carrying capacity should be slightly lower if you drive with a recommended tire pressure.
Experts in the field suggest that it is crucial to choose tires with an index at least the same as that of genuine tires (Original Equipment ones). A higher index is also allowed but in no case is a lower one. In case you bought a vehicle without OE tires, find out about the carrying capacity index on the placard on the driver's side doorjamb. If you have the owner's manual, you can certainly find these specifications in it.
Choosing a set of tires with a carrying index lower than the OE ones leads to a chain of adverse effects. The tires will always be in an overloaded state and will not cope with the technical demands of the car body. Their condition will deteriorate much sooner. In the same context, the risk of road accidents increases following a blowout, especially at high speed.
Speed Rating Chart
Like the carrying capacity index principle, the speed rating tells you the speed limit at which the tire performs safely on a perfect road. To avoid confusion between speed and load indexes, manufacturers use letters to mark the speed rating. The car manufacturing industry is guided by an international standard in which everyone knows what the speed rating is for each letter. In our case, the "Y" is a phenomenal rating that allows a safe drive at a speed of up to 186 miles per hour.
Keep in mind that the speed rating is an index that suggests optimal driving on perfect roads. But in reality, the roads are not exactly perfect, and driving at these speed limits might not be quite safe. We recommend drivers be alert and wise when experimenting with speed limits. 2
Speed Rating Speaks Not Only About Speed
The speed rating index speaks about the safety and stability of the tires not only in certain speed circumstances. It also influences other aspects. These include breaking, cornering grip, heat build-up, and steering responsiveness.
Breaking tire ability
If the tire indicates a low rating, braking will take more time. For example, let's take 2 tires to test them at a speed of 100 km/h. One tire is S-rated, and the other is V-rated. While the V-rating tire stops dynamically, the S one continues to brake at about 25 km per hour. In exceptional situations, this braking delay can play a crucial role in the safety of those involved in risky circumstances. You can check how different tires brake differently on wet roads for optimal performance.
Cornering grip
Tires with a lower rating are prone to deformation while cornering. Deformation, in fact, is possible during braking as well. During turns, the grip becomes tremendously worse, as the contact patch area changes its shape.
Heat build-up
The higher the rating, the more reinforcements and special compounds the tire has to maintain a great flex under pressure. In contrast, those with a low rating are subject to heat build-up because they flex much harder. As a result, it is not surprising that the tire deteriorates much faster over time or undergoes a blowup.
Steering responsiveness
Tires with a higher speed rating offer better overall vehicle responsiveness. The steering requires no effort at all and tire response is immediate at small degrees of steering. In contrast, the response of tires with a low rating is not impacted as long as you drive within the stipulated speed limits. Once you cross the limits, the responsiveness gets harder and harder.
Some Speed Rating Chart Deviations
You probably noticed that the speed rating chart comes in alphabetical order. Still, there are some exceptions that you have to take into account, to know how to read it correctly.
According to the alphabet, H comes after G. But not in the case of the speed rating chart. You will find the H rating between the letters U and V.
Another irregularity refers to the lack of the I, X, and O letters. In some charts, you may not find the letter P.
Some tire manufacturers use the letter Z to actually refer to Y and W speed ratings on the tire sidewalls.
Taking into account that the speed rating system was invented in Europe, it is mainly read based on kilometers per hour, not miles per hour.
Final thoughts
These are the basic criteria you need to know about load and speed ratings. Traditionally, I will end the article with some blitz questions and answers, to memorize the information better.
What speed rating do I need?
First of all, you need to know what the car manufacturer recommends. You can find this information in the owner's manual. So, you must follow his recommendations regarding speed and load ratings. It's not a problem if you go a little higher in terms of speed and load rating. It is important not to reduce the minimum rating recommended by the manufacturer. This will lead to the degradation of the tires and the severe compromise of the vehicle's performance.
What happens if I load the car more than the load rating allows?
Exaggerated load on the tires severely affects their structure. They will wear out much faster. At the same time, you risk undergoing a blowout, especially at high speed or on bad roads. Another side effect is the compromised vehicle's traction on the road.
What happens if I mix tires with different speed ratings?
Manufacturers advise against such combinations. It is much safer for all tires to have the same speed and load ratings. If the situation requires otherwise, then keep in mind that you must fit tires with the same speed rating on the same axis. Also, you must put tires with a lower speed rating on the front axle. The speed you must respect is that of tires with a lower rating.