There are three things to understand when buying new tires for your car: size, speed rating, and load index. Making the correct selection is very important to maintain optimal performance, safety, and fuel efficiency. Fortunately, all of this information is easy to find.
You can locate the size of your tires on the driver’s door jamb, inside the fuel door, or inside the glove compartment. Alternatively, you can consult your vehicle documentation and tire manufacturer specifications.
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When considering tire size, there are two simple rules to follow.
Firstly, the replacement tires should have the same size, load index, and speed rating specification as specified by the vehicle or tire manufacturer.
Secondly, the replacement tires must never be of a smaller size or lower load-carrying capacity than the original specification.
The type of vehicle is also important: Standard passenger cars, SUVs, and electric vehicles have different design characteristics that influence the optimal tire size, construction, and performance.
For example, electric vehicles typically carry more weight due to their battery packs and produce instant torque, which places unique demands on the tires. For electric vehicles, it is especially important to ensure that both the tire size and the Load Index strictly match the vehicle manufacturer’s specifications, as the higher vehicle weight and specific load requirements may in some cases require the use of HL (High Load) tires.
SUVs and light trucks have different weight distribution, vehicle height, and typical usage conditions. As a result, they often use tire sizes with wider widths and different load/speed rating combinations to optimize handling, stability, and durability. In all cases, always consult the vehicle manufacturer’s specifications to choose the right tire.
Tire size describes the physical dimensions and key performance limits of a tire. It tells you how wide the tire is, how tall the sidewall is, what wheel it fits, and how much load and speed it can safely handle. This information is shown as a standardized code on the tire’s sidewall.
But what do these letters and numbers on the sidewall mean? Here’s all the important information for your tire size quickly decoded:
There can be additional markings on the sidewall of your tires , which are not relevant for checking your tire size. Please refer to the linked guide for more information.
Back to our example from the image above. A tire size like 225/45R18 breaks down like this:
Together, these elements ensure that the tire fits the wheel correctly, supports the vehicle’s weight, and performs safely at the vehicle’s intended speeds.
The documentation supplied with your vehicle outlines the recommended tire sizes for your car. Compatibility is determined according to its outer dimensions – diameter/rolling circumference and width – and labeled as a standardized tire size indicator on the sidewall.
Tire size has a direct impact on how a vehicle behaves on the road. It influences safety, efficiency, comfort, and overall driving dynamics. Choosing the correct tire size ensures that the vehicle performs as intended by the manufacturer.
Tire size plays a crucial role in braking performance and overall safety. Wider tires generally provide a larger contact patch with the road, which can improve grip and shorten braking distances on dry surfaces. However, they may also increase the risk of hydroplaning in wet conditions.
Tires with a larger overall diameter can affect braking response, as they may alter the effective gearing and rotational inertia.
Using incorrect tire sizes can lead to longer stopping distances and reduced effectiveness of safety systems.
The size and proportions of a tire significantly influence steering response and vehicle handling. Tires with a lower aspect ratio (shorter sidewalls) typically provide more precise steering and better cornering stability, as they flex less under load. In contrast, tires with a higher aspect ratio offer more flexibility, which can make the ride more comfortable but reduce steering sharpness.
Wider tires can enhance grip in cornering but may also make steering feel heavier and less responsive in certain conditions.
Tire size also affects rolling resistance, which has a direct impact on fuel efficiency and CO₂ emissions. Wider tires and larger diameters generally increase rolling resistance, meaning the engine has to work harder to maintain speed. Narrower tires with optimized tread designs typically reduce rolling resistance and can improve fuel economy.
Choosing a tire size that aligns with the manufacturer’s recommendations helps ensure optimal efficiency and lower emissions.
Tire size can influence how much road noise a vehicle produces, though it is only one of several contributing factors. This noise results from the interaction between the tire and the pavement surface. Tire characteristics such as tread pattern, rubber compound, construction, and size all influence the noise emitted by the tire.
Larger, wider tires tend to create more tire-road contact and more complex vibrations, which can increase noise. Tires with a higher sidewall (higher aspect ratio) can absorb road imperfections more effectively, resulting in a smoother and more comfortable ride.
Incorrect tire size can lead to uneven wear and reduced tire lifespan. If the tire size does not match the vehicle’s specifications, the load distribution and contact with the road may be suboptimal.
Wider tires may wear more quickly if not properly inflated, while tires with incorrect diameter can affect suspension geometry and alignment, leading to irregular tread wear. Maintaining the correct tire size, along with proper inflation and alignment, helps ensure even wear and maximizes tire longevity.
Also, the tire size must comply with the vehicle’s requirements regarding speed rating and load index. Together, these two factors make up the service description for a passenger car tire. It must correspond to the information provided in the vehicle documents.
Load index
The load index rating is the maximum capacity of a passenger car tire, calculated from the maximum permissible axle load range between two tires.
Speed index
The speed index rating is also a relevant factor when choosing a tire. The tire’s speed capability must be at least equivalent to the top speed of the vehicle. Its speed rating indicates the maximum permissible speed (at full load) of a tire, and whether it’s appropriate for your car.
When replacing tires, it’s common to wonder whether you can use sizes different from the original manufacturer’s recommendation.
You can use alternative tire sizes – but there are important technical and legal conditions you must follow to ensure safety and compliance. For example, make sure that any alternative size you consider falls within the range that is compatible with your vehicle’s design.
When comparing two sizes, check the diameter, width, and aspect ratio of the tires. There are online tools that let you input your current tire size and the size you are considering. They then calculate and display the differences. Many tire size calculators help identify options with a similar overall diameter and rolling circumference to your original tires.
In addition to technical compatibility, it’s essential that the alternative size is officially permitted for your vehicle. Most modern vehicles have a list of approved tire sizes documented in the vehicle’s registration certificate or in the Certificate of Conformity (CoC). Using a tire size outside of what’s listed in your vehicle documentation may require official approval or inspection by a qualified professional.
If in doubt, ask a tire specialist. They can help ensure that the new tires not only fit physically but also meet the required load index and speed rating specifications.
Tire size selection can also vary depending on the seasonal tire type you choose. Summer tires, winter tires, and all-season tires are designed for different temperature ranges and driving conditions. This can influence both the recommended tire size and service description.
In many cases, manufacturers approve different tire sizes for winter use than for summer driving. Winter tire setups often feature a narrower width and a higher aspect ratio, which helps improve traction on snow and slush. They may also have a lower speed index than the vehicle’s maximum speed. In many regions, this is permitted as long as the tire’s speed rating still meets the minimum legal requirements and the driver is sometimes required to be clearly informed (e.g., via a visible sticker), depending on the market.
All-season tires, which combine characteristics of summer and winter tires, are usually offered in the same sizes as standard summer tires, but not all sizes are available for every vehicle.
When talking about tire size, it’s also critical to understand the role of tire air pressure and how it interacts with tire size to affect safety, performance, and tire life. The correct inflation pressure is specified by the vehicle manufacturer based on the original tire size, load, and vehicle dynamics and is usually found on a sticker inside the driver’s door jamb or in the owner’s manual.
When you change to a different tire size, you may also need to re-evaluate the recommended tire pressure. Tire pressure monitoring systems (TPMS) may also need to be recalibrated or reset after changes in size or pressure. While the manufacturer’s pressure recommendation for the original tire size is always the starting point, alternative sizes with different load capacities or overall diameters can require adjustments.
Maintaining optimal tire pressure is essential because it directly influences several key performance and safety aspects:
For all these reasons, when selecting a new tire size, you should always check and adjust the tire pressure. Consult a professional if you are unsure.
Select your tire size in strict accordance with vehicle documents and tire manufacturer recommendations. If the combined wheel and tire diameter aren’t correctly matched, there may be drastic differences in ride height and speedometer readings.
For example, fitting 17-inch-diameter wheels to a car that is designed to have 14-inch-diameter wheels will change the ride height. That means the car body will be higher off the ground. There will also be a slight increase in top speed, and the speedometer will no longer be accurate. Last but not least, the car will accelerate slower.
Switching to different tire sizes can only be done in compliance with all legal requirements and regulations – plus the specifications provided by the vehicle, wheel, and tire manufacturers. At the very least, the wheel must rotate freely without interference and the tire’s adequate load capacity must be observed.
The internal construction of tires fitted on any vehicle must either be radial or bias-ply. Fitting mixed tire constructions (bias-ply and radial together) for cars, caravans, and other light trucks is unsafe and illegal. The only permissible exception is the use of a spare tire for an emergency.
Concerning the choice of wheels or rims, the same guidelines apply. Motorists must use the standard wheels or rims approved by the vehicle manufacturer as recommended.
For optimal safety and performance, replace all four tires at the same time. While it is possible to switch out less than four tires at the same time, you should follow a couple of guidelines regarding tire size.
If only replacing one or two tires, for example, ensure that each one is the same size and has the same load index and speed rating as specified by the car manufacturer.
When replacing only two tires, fit both of them to the wheels on the rear axle. The reason for this is that the newer tires will have much better grip, particularly on wet roads, and maintaining grip on the rear axle is crucial for safety reasons.
Replacing only a single tire is not advisable, as it can impact vehicle suspension or transmission and produce excessive wear on the tire tread. But if replacing only one tire is unavoidable, then pair the single replacement tire with the tire that has the deepest tread depth, then fit both to the rear axle.
Learn more about when it's time to replace your tires in our guide.
Sizes vary widely based on vehicle type, but there are several sizes that are especially common for everyday passenger cars. Some of the most frequently encountered tire sizes include:
These tire sizes offer a balanced mix of performance, comfort, and fuel efficiency, making them a popular choice among manufacturers and drivers alike.
No, tire sizes are not exactly the same worldwide, although they are largely standardized internationally. Most passenger car tires use the metric tire sizing system, which is recognized globally and regulated by international standards organizations. This means that a size such as 205/55R16 represents the same physical dimensions whether the tire is sold in Europe, North America, or many other regions. However, there can be a few specialty or regional labeling differences.
There isn’t a single “magic size,” but narrower tires with low rolling resistance generally improve fuel economy compared to wider or oversized tires. Narrow tires reduce the amount of rubber in contact with the road, which lowers rolling resistance and means the engine doesn’t have to work as hard.
Yes, larger tires can be beneficial for off-road vehicles, but with trade-offs. Bigger tires typically provide:
