Buying new tires for a car often turns into a real quest, where instead of the desired characteristics, the driver is faced with a mysterious code on the sidewall. The numbers and letters following the tire size contain critical information about its physical capabilities. Exactly load index and speed index determine whether the wheel can withstand the weight of the car during sudden acceleration or when the trunk is fully loaded. Ignoring these parameters can lead to cord failure and an emergency on the route.
Many car owners mistakenly believe that it is enough to know only the diameter of the disk and the width of the profile. However, the labeling 91V or 95H says much more than just the tire model. This is a product technical data sheet approved by the manufacturer after a series of rigorous tests. Understanding these codes allows you not to overpay for unnecessary characteristics, but also not to skimp on your own safety by choosing tires that are physically unable to work in your operating mode.
In this article we will look in detail at how to read these codes, why you cannot install tires with a lower index than the factory recommends, and how seasonality affects the choice of speed limit. You will learn to independently determine the suitability of tires for your car, based on dry facts and technical standards.
Where to look and how to read the markings on the sidewall
Basic information about the tire parameters is printed on the sidewall in large raised font. Usually this is a combination of numbers and letters, immediately following the indication of the profile width and aspect ratio (series). For example, in the entry 205/55 R16 91V we are interested in the last two characters before the letter V. The number 91 is the load index, and the letter V — speed index. You can find this marking on the outside or inside of the wheel, depending on how the tire is mounted on the rim.
It is important to understand that the load index is not the maximum weight that one wheel can withstand in static conditions, but the maximum load when moving at a certain speed. This is a dynamic parameter. If you see the marking Load Range or letters XL (Extra Load), this means that the tire is reinforced and has an increased load capacity index compared to a standard model of the same size.
Always check the markings on all four wheels. Sometimes cars may be equipped with tires with different speed indexes, which is especially typical for cars after repair or change of owner.
The speed index should not be confused with the recommended driving speed. The specified value means that the tire can briefly withstand this speed at full load. However, prolonged driving at the limit values leads to overheating of the rubber compound and accelerated wear. For comfortable and safe operation, it is recommended to choose tires with a margin in both parameters.
Load index: table of values and weight calculation
The digital code indicating the load capacity is one of the most important safety parameters. It shows the maximum weight that one tire can carry when driving at the maximum speed corresponding to its index. An error in choice can lead to deformation of the frame, swelling of hernias, or even explosion of the wheel under load. Index values typically range from 60 to 120 or higher for cars and light trucks.
To make the correct selection, you need to know the curb weight of the car, add the weight of passengers, luggage and fuel, and then divide the resulting amount by four (the number of wheels). However, simply dividing the mass in half or by four is also not always correct, since the load on the axles is distributed unevenly. Typically, the rear axle carries a smaller percentage of the weight, but all wheels should have a margin of safety.
| Index | Load (kg) | Index | Load (kg) | Index | Load (kg) |
|---|---|---|---|---|---|
| 85 | 515 | 91 | 615 | 97 | 725 |
| 86 | 530 | 92 | 630 | 98 | 750 |
| 87 | 545 | 93 | 650 | 99 | 775 |
| 88 | 560 | 94 | 670 | 100 | 800 |
| 89 | 580 | 95 | 690 | 101 | 825 |
| 90 | 600 | 96 | 710 | 102 | 850 |
There is a common misconception that if the load index is higher than required, then the tire will be stiffer and will hold the road worse. In fact, the carcass of a higher index tire is actually stiffer, which may improve handling when fully loaded, but may decrease slightly when the car is empty. However, install tires with index below factory made is strictly prohibited.
☑️ Checking load capacity
Speed index: letters and restrictions
The Latin letter following the load number indicates the maximum speed that the tire can withstand. This parameter is directly related to the heat resistance of the rubber compound and the strength of the frame. When the speed limit is exceeded, the tire begins to heat up, the pressure inside increases, and delamination of the layers may occur. The designations are in alphabetical order, but with some letters missing (for example, there is no I, O, Q at the beginning of the row for passenger cars).
The most common indices for civilian vehicles are H (up to 210 km/h), V (up to 240 km/h) and W (up to 270 km/h). Winter tires often have a lower speed index, e.g. T or H, since operating conditions in winter do not imply racing speeds, and the rubber composition is focused on elasticity in cold weather, and not on heat resistance when heated.
⚠️ Attention: Installing tires with a speed rating lower than that recommended by the vehicle manufacturer makes operating the vehicle illegal and dangerous. The insurance company may refuse to pay in the event of an accident if it is proven that the tires did not meet technical requirements.
If you choose winter tires with a lower speed index than summer tires (which is allowed provided that an appropriate speed limiter is installed in the cabin or the driver consciously limits it), you need to be extremely careful. On dry asphalt in warm weather, such tires can behave unpredictably at high speeds due to rapid wear and overheating.
Seasonal features: summer, winter and all-season tires
Seasonality has a direct impact on permissible loads and speed limits. Summer tires are designed to withstand high road temperatures, so their speed index is often higher. Winter tires must remain soft in the cold, which is achieved through a sensitive chemical composition, which, alas, degrades faster when overheated. That's why winter tires are often marked Q, T or H, even for powerful cars.
All-season tires are a compromise solution. They do not have outstanding characteristics either in the heat or in severe frost. When choosing an all-season vehicle it is critical to focus on the climate of your region. If temperatures often drop below -10°C in winter and rise above +25°C in summer, general-purpose tires will wear out faster and perform less efficiently than specialized tires.
There is an opinion that for winter you can take tires with a margin of width, but a lower speed index. This is acceptable, but only if you are willing to realistically not exceed this limit. For example, by installing tires with the index T (190 km/h) for a car that easily accelerates to 220 km/h, you are bound to forget about driving fast on the autobahn in winter.
Why are winter tires narrower than summer tires?
It is often recommended to install winter tires narrower (smaller width) in order to increase the pressure in the contact patch. This allows the tire to better “cut through” the snow slush and reach hard surfaces, improving traction. In summer, a wide contact area is important for better braking on dry asphalt.
Special markings: XL, RunFlat and C
In addition to the basic indices, you can find additional symbols on the sidewall that affect the performance characteristics. Marking XL (Extra Load) or Reinforced indicates reinforced sidewall and cord construction. Such tires can withstand greater pressure and load, which is important for heavy crossovers, electric vehicles (due to the weight of the batteries) and cars with a full load.
Technology RunFlat (designated differently by brands: RSC, SSR, ZP) allows you to drive a certain distance on a flat tire. However, such tires have very stiff sidewalls and, as a rule, a higher load index. Installing regular tires instead of RunFlat is only possible if you have a spare tire or a repair kit, but the reverse replacement (RunFlat instead of regular) requires checking the suspension for compatibility.
For light commercial vehicles and minibuses there are tires with the index C (Commercial). They have a significantly higher load index and a multi-layer frame. It usually makes no sense to install tires with index C on a passenger sedan - they will be too hard and noisy, although technically they will withstand the weight of the car with a large margin.
Common mistakes when choosing and installing tires
One of the most common mistakes is installing tires with different speed ratings on the same axle. This leads to uneven wear, the vehicle pulling to one side and unstable braking behavior. At different axles, differences are allowed (for example, wider and faster at the rear), but only if this is provided by the car manufacturer.
Also, drivers often ignore the production date, buying “fresh” tires with a low index, thinking that they are better than old ones with a high index. But the age of the tire (no more than 5-6 years from the date of production) is more important than formal compliance with the numbers. Old rubber loses elasticity, cracks and cannot bear the declared load, regardless of the stamped index.
⚠️ Attention: Never use sealants to repair punctures on run-flat or low-profile tires unless specifically prohibited by the tire manufacturer. The chemical reaction can destroy the inner layer and the TPMS pressure sensors.
When buying used tires, pay attention not only to the remaining tread, but also to the presence of repairs. Harnesses and fungal patches can reduce the actual speed and load index of the tire. A retreaded tire is no longer a factory product, and its speed characteristics become unpredictable.
Safety is always a priority over savings: it is better to buy tires that are slightly narrower, but with the correct high load and speed index, than wide ones, but that do not meet the requirements of your car.
Is it possible to install tires with a load index higher than indicated in the documents?
Yes, it is possible and even necessary if you often carry heavy loads. An increased load index indicates that the tire is stronger than the minimum required standard. This will not negatively affect the performance of the suspension or ABS, but will only increase the safety margin of the wheel.
What happens if you exceed the speed index for a short section?
A short-term excess (for example, overtaking) usually does not lead to instant destruction, since engineers include a small margin of safety. However, systematic driving at speeds above the stated index leads to overheating, delamination of the carcass and a sharp decrease in tire life.
How to find out the speed index if the letters on the sidewall are erased?
If the marking is not readable, you can find out the exact index by looking at the model catalog number (usually it is also stamped on the side) on the manufacturer’s website or in specialized online stores. Enter the full size code and tire model range.
Does tire pressure affect the load index?
Yes, directly. The load index is only valid at the recommended pressure. An underinflated tire has less load capacity, runs hotter and wears out faster at the edges. An overinflated tire becomes stiffer and holds the road worse, although it formally can withstand pressure.