How To Read Tire Size and Choose The Right Tires

Written by Thomas Nielsen

September 28, 2023

Tire size is a complex topic. On the sidewall, some of the numerals are listed in millimeters, while others are listed in inches. Additionally, depending on where you drive and how you drive, your car, light truck, or trailer may need a different tire size.

Tire Basics

Understanding tire basics is essential before choosing tire size. The correct tire size for your vehicle can be found in several places, including the tire sidewall, the vehicle’s owner’s manual, and the information plaque located on the driver’s door jamb or inside the gas tank hatch. The owner’s manual provides manufacturer-recommended tire size, load capacity, and recommended inflation pressures, making it the most reliable source for this information. Selecting the right tire size is important for safety, performance, and maintaining your vehicle’s warranty.

Your owner’s manual is the primary and most reliable source for determining the correct tire size, load capacity, and recommended inflation pressures for your vehicle. You can also find this information on the plaque often found on the driver’s glove box door or inside the gas tank hatch. This is the tire size that the vehicle manufacturer suggests.

The numbers and other indicators on the sidewall of your current tires are a fantastic place to start if you’re interested in replacement tires for performance. We help you choose the correct tire size for your car and understand how tire sizes affect performance with 4x4 and off-roading vehicles.

The tire size sequence

When you consider the tire identification number, you will notice 7 different pieces of information regarding size tire. The tire size meanings go as such:

The tire type, which refers to passenger tires, light truck tires, and special trailer tires.
The tire width is a three-digit number. The first number in the tire size indicates the tire width in millimeters, measured from sidewall to sidewall.
The tire’s aspect ratio is the combination of car sizes and the two-digit aspect ratio. The tire's aspect ratio is the ratio of the tire's height (sidewall) to its width, and this ratio determines the tire's profile and affects handling and ride comfort. Typically, lower aspect ratio tires have low profile or tire sidewall. This refers to the measurement from wheel rim to top of the tread. For example, if the aspect ratio is 65, the tire’s sidewall is 65% as high as the tire’s width. Tire makers calculate the lower or higher aspect ratio tires in inches. The tire's cross section is used to determine the sidewall height and overall tire profile.
The construction type. The construction type is indicated by a letter, such as 'R' for radial, which is the most common type. This letter shows the internal construction maintaining the tire's structure. The tire's construction refers to how the tire’s internal ply cords are arranged. Radial tires have better road grip, ride comfort, durability, and gas mileage. The tire’s internal ply cords visible in the tire’s cross section are perpendicular to the direction of travel. Another construction type is bias constructed tires, also known as cross ply tires, where the plies are crisscrossed and not perpendicular to the direction. Cross ply tires are commonly used in motorcycles and trailers.
Wheel diameter appears in inches. Ideally, the tire size must fit around the wheel. If you wish bigger tire size and tire width, you may need bigger wheels.
Load index number corresponds to the tire load index. This shows how much weight a tire can carry when inflated to the maximum air pressure rating.
Speed rating letter corresponds to the top speed rating that’s safe for travel. Higher speed ratings provide more control at faster speeds. The maximum vehicle speed capability is no more than the lowest speed rating when all tires are mounted. Of course, you must follow legal speed limits when you wish to maintain vehicle speed capability. The maximum speed is always considered by the manufacturer to calculate these. Beware; speed rating refers to physical speed rather than legal speed. But when considering speed rating, the driver must always review the legal compliances.

Passenger vehicles, light trucks tires and other vehicles can have tire size charts expressed in P metric or euro metric tires. P metric is the standard established by the tire and rim association for a passenger car.

Tire Construction and Type

When it comes to choosing the right tires for your vehicle, understanding tire construction and type is just as important as knowing the correct tire size. The way a tire is built—its internal construction—directly impacts its strength, durability, and how it handles on the road. Most modern vehicles use radial tires, which feature internal ply cords running radially from bead to bead. This radial construction means better road grip, improved ride comfort, and enhanced fuel efficiency, making them the top choice for both passenger vehicles and light trucks.

Tire type is indicated by a letter at the start of the tire size sequence on the tire’s sidewall. For example, “P” stands for passenger tires, designed for everyday cars and smaller vehicles, while “LT” designates light truck tires, which are built to handle heavier loads and are ideal for vehicles capable of carrying heavy cargo or towing trailers. Light truck tires have reinforced construction to support higher load capacity and are essential for light duty pickup trucks, vans, and SUVs that frequently haul or tow.

The tire’s construction and type also influence its speed rating and load index. The speed rating, shown as a letter at the end of the tire size, tells you the maximum speed capability the tire can safely handle. The load index, a number found just before the speed rating, indicates how much weight the tire can support when properly inflated. For vehicles that regularly carry heavy loads or tow, selecting tires with a higher load index and the correct speed rating is crucial to maintain vehicle speed capability and ensure safety.

Another important factor is the tire’s width and aspect ratio. The width of the tire, measured in millimeters, affects traction and stability, while the aspect ratio (the ratio of the tire’s height to its width) influences the tire’s profile and ride quality. Lower aspect ratio tires often provide sharper handling and a sportier feel, while higher aspect ratio tires offer a smoother ride and are better suited for vehicles that prioritize comfort or need to handle rougher roads.

The Uniform Tire Quality Grading (UTQG) system, which rates tires for treadwear, traction, and temperature resistance, can help you compare the quality and performance of different tire types. Tires with higher UTQG grades, such as “AA” for traction, generally offer better performance and safety characteristics, especially in challenging driving conditions.

When shopping for new tires, always check the information on the tire’s sidewall to ensure you’re selecting the correct tire size, type, and construction for your vehicle’s needs. Matching the right tire construction and type to your vehicle—whether it’s a passenger car, light truck, or SUV—ensures you get the best combination of safety, performance, and durability, whether you’re commuting, carrying heavy cargo, or towing trailers. By considering all these factors, you’ll be well-equipped to choose new tires that keep you safe and your vehicle performing at its best.

The uniform tire quality grading

The UTQG appears on passenger tire for summer and all season categories. Winter tires and motorcycle tires are excluded.

The UTQG refers to:

Treadwear: The wear rate under controlled conditions. This can give you an indication of the longer tread life under normal conditions.
Traction grade: This shows wet braking traction test.
Temperature grade: This shows the tire's resistance to heat under controlled conditions. The temperature grade is established under proper inflation with no excessive load capacity and no excessive speed.

The UTQG is calculated referring to the standard load index for the vehicles. There is no indication of load index capacity for a car or truck. It also doesn't show any speed rating calculations.

tire tread

How to choose the correct tires

Modern vehicles come with an owner’s manual that can be reviewed by tire manufacturers to ensure your vehicles are equipped with the right tire sizes. Understanding size tires is essential for ensuring compatibility with your vehicle's wheels and requirements. It is crucial that the tire complies with the recommendations as switching to new tires that don’t meet the recommended tire’s width, tire sidewall aspect ratio, tire size, load index, and even rim diameter could affect the vehicle handling performance advantages. After checking the load index, always verify the specific load capacity listed for each tire to ensure it meets your vehicle’s needs, as the load index number alone may not reflect the exact weight capacity.

Ensuring you've got the right tire for your vehicle

Not all passenger cars are made the same. Therefore, it is essential to refer to the manufacturer's tips regarding your vehicle:

passenger vehicle
light duty pickup trucks
trailers carrying heavy cargo or light cargo
motorcycle
passenger car with specific load capacity
4x4

Understanding the appropriate tire's load index for your passenger tire

Is there a load limit your tire can carry? The answer is yes, and if you try to carry more than the load index rating, your tire could explode. So, you want to ensure you understand the different load capacity for each tire and the maximum load index for your tire type.

Load Index vs. Maximum Load

Load ratings are represented in two ways: load index and maximum load. The load index is a numerical code that corresponds to a specific weight-carrying capacity, while the maximum load is the actual weight in pounds or kilograms that the tire can support when properly inflated. To simplify, the load index is a standardized system that helps you quickly identify a tire's capacity.

Determining Your Vehicle's Load Requirements

To select the right tires, you must first determine your vehicle's load requirements. This involves understanding the Gross Vehicle Weight Rating (GVWR), which is the maximum weight your vehicle is designed to carry safely. You can find the GVWR in your vehicle's owner's manual or on a placard inside the driver's door jamb.

Once you have the GVWR, you'll need to consider the distribution of weight between the front and rear axles. This will help you determine the load rating required for both your front and rear tires. Keep in mind that different vehicles may have different load ratings for their front and rear tires, so it's essential to check your specific vehicle's requirements.

Selecting Tires with Adequate Load Ratings

When choosing tires, ensure that the load rating of each tire matches or exceeds the weight it will carry. It's advisable to select tires with a load rating that provides some margin above your vehicle's actual weight to account for variations in load distribution and to prevent overloading the tires. If you regularly carry heavy loads or tow trailers, you should choose tires with higher load ratings to accommodate the extra weight.

Consequences of Ignoring Load Ratings

Neglecting load ratings can lead to serious safety issues. Tires that are not designed to handle the weight of your vehicle and its cargo may experience accelerated wear, reduced traction, and an increased risk of blowouts or tire failure, especially under heavy load conditions.

How to read tire size for 4x4

Driving a 4x4 brings a whole new set of challenges and will ultimately require you to consider new tires that can address your needs. Among which, load index, maximum speed, and radial construction are crucial aspects. Beware of opting for excessive tire width or tire size which can affect handling.

Load index with 4x4

Does the load index make a huge difference when you're in an SUV? It can completely transform your experience and performance, especially if you consider towing a heavy load. Make sure your tire size and tire type are fully adjusted to the most relevant load index. Your load index must include potential trailer towing, passenger load, additional cargo.

It's worth noting that radial tires are vulnerable to abuse in overloaded vehicles. This could happen with a 4x4 towing a blocked vehicle, for example. Criss-crossed construction is preferred to radial tire. A radial tire is best suited for a passenger vehicle.

The right tire size for 4x4

All terrain, mud terrain and highway terrain are important differentiation when it comes to tire type. For uneven grounds, all terrain and must terrain are preferable. Tire tread and tire's sidewall height may vary depending on the terrain.

Typically, for off-roading, we recommend selecting a manageable tire size that doesn't affect handling, with a rim diameter no bigger than half the tire height, such as 15 or 16-inch wheel diameter for 33s, or 17-inch rim diameter wheel for 35s.

Lower aspect ratio tires typically show a high performance with better stability laterally. So, when replacing your 4x4 tires, pay close attention to the aspect ratio if you wish to change size.

Different speed ratings tires on 4x4?

We strongly advise against fitting tires with a different speed rating from what you need. Lowering the speed rating to anything less than the manufacturer's recommendation can be counterproductive.

We hope you enjoyed reading about how to read tire size. At Rhino USA we are experts in recovery gear and off-road accessories. Make sure to check out our store to gear up for your next off-road adventure!

FAQ's

Can I use tires with a different size than what is recommended by the manufacturer?

While it is possible to use tires with a different size than what is recommended by the manufacturer, it is generally not recommended. Using the wrong size tire can affect the handling and performance of your vehicle, and can also increase the risk of accidents, and cause more blowouts. It's important to always use the tire size recommended by the manufacturer, or to consult with a tire specialist to determine the appropriate tire size for your specific vehicle.

How often should I replace my tires?

The lifespan of your tires will depend on a variety of factors, including the type of driving you do, the weather conditions in your area, and the quality of the tires themselves. As a general rule, most tires should be replaced approximately every six years, regardless of their mileage. Additionally, you should regularly inspect your tires for signs of wear and damage, and have them replaced if they appear to be worn or damaged beyond repair.