Tire sizes may seem like a jumble of numbers and letters, but they are crucial in determining the right tires for your vehicle.
The specific size designation indicates key performance characteristics like width, height, diameter, load capacity and speed rating.
This code unlocks vital factors in acceleration, braking, ride comfort, noise and longevity.
Getting the proper tire size affects handling, safety and efficiency. Undersized tires reduce stability while oversized tires cause interference.
Using the manufacturer’s recommended size ensures ideal interaction with your vehicle’s suspension and drivetrain.
The right fit also keeps speedometer/odometer accuracy.
Tire size calculators help take the guesswork out of decoding sidewall markings.
By inputting your existing size or vehicle specs, these tools recommend compatible new sizes.
They provide a shortcut to finding tires that meet your needs regarding traction, load-carrying, speed capability and overall performance.
Size calculators match your needs with available options to simplify upgrading or replacing.
Tire Size Calculator
Tire Sizing Systems
There are several different tire sizing systems used by tire manufacturers to convey information about a tire’s dimensions and capabilities.
Some of the key sizing systems include:
Metric Tire Sizes
Metric tire sizes provide the width and aspect ratio measurements in millimeters.
For example, a 195/65R15 tire would have a width of 195mm and an aspect ratio of 65%.
Metric sizes are commonly used on European and Asian vehicles.
Percentage Aspect Ratio
The aspect ratio is the height of the sidewall expressed as a percentage of the total width.
So a 65 aspect ratio means the sidewall height is 65% of the tire’s width.
Lower profiles like 35 or 40 have shorter sidewalls, while higher profiles like 70 or 80 have taller sidewalls.
Ply Rating
Ply rating refers to the number of plies, or layers of rubber-coated fabric, in a tire carcass. More plies generally indicates a stronger and heavier tire construction.
Common ply ratings are 4, 6, and 8 ply tires. Ply rating has declined in importance with the move to radial tire construction.
Load Index
The load index is a numerical code that specifies the maximum load a tire can carry at a defined inflation pressure.
For example, a 91 load index indicates a maximum load of 1,356 lbs.
Load index provides an easy way to determine if a tire meets your vehicle’s weight requirements.
UTQG (Uniform Tire Quality Grading)
The UTQG rating grades tires under three parameters – treadwear, traction, and temperature resistance.
It provides a standardized way to compare tire quality and durability.
However, the ratings are provided by tire manufacturers, not an independent agency.
How to Read Tire Sidewalls
Properly interpreting the markings on your tire’s sidewall is crucial for understanding its size and specifications.
While tire sidewall markings may seem like a mysterious jumble of letters and numbers at first glance, they actually contain a wealth of information once you know what to look for.
The sidewall will display the tire’s full size description, which indicates the tire’s width, aspect ratio, and wheel diameter.
It will also show the load index and speed rating, which specify how much weight the tire can support and its maximum speed capability.
Here are the key components to look for when reading tire sidewall markings:
- Tire width – This 3-digit number gives the width of the tire in millimeters from sidewall to sidewall. So a tire marked 225 would be 225mm across.
- Aspect ratio – This 2-digit number shows the sidewall height as a percentage of the tire’s width. So if a tire has an aspect ratio of 60 on a 225mm wide tire, the sidewall height is 60% of 225mm, or 135mm.
- Rim diameter – This 2-digit number is the diameter of the rim that the tire is designed to fit, measured in inches. A marking of 17 indicates the tire fits a 17-inch diameter wheel.
- Load index – Represented by a 2 or 3-digit code, the load index specifies the maximum load carrying capacity the tire is certified to handle. Higher numbers mean higher weight capacities.
- Speed rating – Indicated by a letter code like V or ZR, this shows the maximum safe speed that the tire is designed to handle. Higher speed ratings correspond to higher maximum speeds.
So a tire marked 225/60R17 102V tells you the tire is 225mm wide, has a 135mm sidewall height (60% of 225mm), fits a 17-inch wheel, has a 1,200 kg load capacity, and is rated for speeds up to 149 mph.
Learning how to decode tire sidewall markings empowers you to understand the key specifications of any tire.
Tire Width
Tire width is exactly what it sounds like – the width of the tire from sidewall to sidewall.
It is one of the most important dimensions of a tire and plays a crucial role in tire performance.
Tire width is measured in millimeters. Typical passenger car tire widths range from 135mm to 285mm, while truck and SUV tires can range from 225mm all the way up to 365mm or more for specialized off-road tires.
Wider tires offer a larger contact patch which provides more grip, better cornering, and improved braking.
This makes them well suited for performance cars. The downside is wider tires have higher rolling resistance which decreases fuel economy.
Narrower tires have less rolling resistance so they can improve fuel efficiency.
However, their smaller contact patch reduces grip which is not ideal for sports cars or fast driving.
Narrow tires are best for lighter vehicles focused on good gas mileage.
For most drivers, choosing a tire width in the middle of the range – between 205mm to 255mm – provides a good balance of grip, comfort, noise, and fuel economy.
Consider your specific vehicle, typical driving, priorities, and local road conditions when selecting tire width.
Consult your owner’s manual for any manufacturer recommendations on appropriate tire widths.
Aspect Ratio
The aspect ratio of a tire refers to the ratio of the tire’s section height to its section width.
It is typically represented as a percentage or ratio.
For example, an aspect ratio of 60 means the tire’s section height is 60% of its section width.
The aspect ratio impacts the tire’s ride quality, handling characteristics, and appearance.
Lower aspect ratio tires generally have shorter sidewalls and provide less cushioning from impacts.
This tends to produce a firmer ride but enhanced steering response and cornering.
Higher aspect ratio tires have taller sidewalls which absorb more shock and imperfections in the road, delivering a softer, more comfortable ride.
They provide less precise steering and handling compared to low profile tires.
Common aspect ratios for passenger vehicles range from around 50 to 70.
Performance cars and sports sedans tend to use aspect ratios in the 40s or even 30s for maximum handling performance.
SUVs, trucks, and off-road focused vehicles often use aspect ratios of 75 or higher to improve ride quality and traction over rough terrain.
In general, an aspect ratio of 60-65 offers a good balance of ride comfort, noise isolation, and handling for most drivers.
The aspect ratio also impacts the tire’s overall diameter, with lower ratios producing shorter tire heights.
This must be considered when selecting new tires to ensure proper speedometer calibration and gearing.
Rim Diameter
Rim diameter is one of the key measurements that makes up the tire size. It refers to the diameter of the wheel that the tire is designed to fit.
The wheel diameter is represented by the last number in the tire size. For example, in a size of P215/65R15, the 15 indicates a wheel diameter of 15 inches.
Some common wheel diameters for passenger vehicles are:
- 13 inches – Typically found on compact cars and some older vehicles. Provides a smoother ride but less grip.
- 14 inches – Used on midsize sedans and some small SUVs. A good balance of ride comfort and handling.
- 15 inches – The most common diameter on modern cars. Provides good grip and performance.
- 16 inches – Used on larger sedans and SUVs. Prioritizes grip and handling over ride comfort.
- 17 inches – Found on sports cars and performance vehicles. Maximizes grip and handling capabilities.
Larger wheel diameters typically provide better handling and braking performance due to the larger brake rotors they can accommodate.
However, they may result in a harsher ride.
Smaller wheel diameters tend to provide a more comfortable ride, but compromise on performance.
It’s important to choose a wheel diameter that matches the vehicle manufacturer specifications.
Installing a different diameter wheel can negatively impact the speedometer accuracy, gearing, and handling.
When selecting new tires, the wheel diameter must match the existing wheels on your vehicle.
Even a difference of an inch can prevent proper fitment.
Referencing your vehicle owner’s manual or existing tire sidewall will provide the diameter needed.
Speed Rating
Most tires display a speed rating, which indicates the maximum speed capability of the tire.
Speed ratings are designated by letters that correspond to standardized speed limits.
- M – Up to 81 mph
- Q – Up to 99 mph
- R – Up to 106 mph
- S – Up to 112 mph
- T – Up to 118 mph
- U – Up to 124 mph
- H – Up to 130 mph
- V – Up to 149 mph
- W – Up to 168 mph
- Y – 186 mph and above
Higher speed rated tires typically have stiffer sidewalls and tread to handle the stresses of high speed driving.
Going with a lower speed rating than your vehicle’s top speed is generally not recommended for safety reasons.
Exceeding a tire’s speed rating can cause overheating, tread separation, or a blowout. Most passenger vehicles will require at least an S or T speed rating for their tires.
High performance sports cars may need Z, W, or even Y ratings depending on their capabilities.
When selecting new tires, it’s important to get ones with an equal or higher speed rating than the OEM tires that came on your vehicle.
Refer to your owner’s manual or vehicle documentation to determine the correct speed rating.
Choosing the right rating helps ensure your tires can handle the maximum speeds your vehicle is designed to reach.
Load Index
The load index is a numerical code that indicates the maximum load a tire can support at a defined inflation pressure.
This index is especially important for ensuring you choose a tire that can handle the weight of your vehicle.
Load indexes typically range from 60 to 120. The higher the load index, the more weight the tire is designed to support.
Here’s an overview of common load index ratings and the maximum loads they correspond to:
- 60 = 551 lbs
- 65 = 616 lbs
- 70 = 670 lbs
- 75 = 733 lbs
- 80 = 793 lbs
- 85 = 853 lbs
- 90 = 907 lbs
- 95 = 979 lbs
- 100 = 1,102 lbs
- 105 = 1,189 lbs
- 110 = 1,278 lbs
- 115 = 1,370 lbs
- 120 = 1,471 lbs
So if your vehicle weighs around 3,500 lbs, you’ll want to look for a tire with a load index of at least 105 to safely support that weight.
Going with too low of a load index means the tire will wear faster and potentially fail.
The maximum inflation pressure also factors into the load handling capacity.
A higher inflation pressure allows the tire to carry more weight at a given load index.
So proper inflation is key to maximizing a tire’s load capacity.
Knowing the load index and inflation pressure will ensure you get a tire engineered to handle the demands of your vehicle.
This helps prevent blowouts or other failures that can occur when overloading a tire beyond its specifications.
Checking the load index is a simple way to choose the right tire for your needs.
Using a Tire Size Calculator
A tire size calculator is an invaluable tool for understanding tire sizing and checking fitment.
With so many complex tire size measurements, a calculator makes it easy to decode tire sizes and see how they compare.
Here’s a walkthrough for using an online tire size calculator:
- Go to a tire size calculator website or app. There are many good options available for free online.
- Select your current tire size by entering the sizing information from your tire sidewall. For example, enter “245/35R19”.
- The calculator will decode each component of the tire size:
- 245 is the width in millimeters
- 35 is the aspect ratio (% of sidewall height to width)
- R means radial construction
- 19 is the diameter of the wheel in inches
- Enter the alternative tire size you want to compare to. For example, enter “255/30R19”.
- The calculator will decode the new tire size and show you how the measurements differ. In this example, the new tire is wider and has a lower-profile sidewall.
- Use the calculator to see how the overall diameter changes between your current and alternate tire size. This is crucial for maintaining proper speedometer and odometer accuracy.
- Check whether the new size will fit on your existing wheels without rubbing or requiring modifications.
- Use all of this information to determine if the new tire size will work for your vehicle and needs.
A tire size calculator makes it easy to compare sizes and see the actual measurements.
This takes the guesswork out of choosing new tires and ensures optimal fitment.
Taking a few minutes to use a tire calculator will give you confidence that your new tires will fit properly.
Finding the Right Tire Size
When choosing new tires, you want to make sure you select the optimal size for your specific vehicle and intended use.
Here are some tips for choosing the right tire size:
- Check your owner’s manual – The owner’s manual for your vehicle will indicate the recommended tire size from the manufacturer. This is usually the ideal size to maintain proper speedometer and odometer accuracy. Going with a different size could affect wheel speed sensors.
- Consider OEM tire sizes – Looking at the original tires that came on your vehicle is a good starting point. While you don’t have to stick with the same exact size, choosing a similar size will ensure proper fit and performance.
- Take into account wheel width – The width of your wheels will impact the ideal tire size. Wider tires typically require wider wheels for proper mounting and inflation. Consult wheel and tire experts for the optimal pairing.
- Factor in load capacity – Look at the vehicle’s weight, average passenger and cargo loads, and your intended usage to determine appropriate load capacity for new tires. Insufficient load capacity can lead to tire failure.
- Check clearance – Make sure the new tire size will fit properly within the wheel well at full lock and suspension travel. Larger tires could rub at extreme angles.
- Consider speed rating – Choose an adequate speed rating for your needs. Higher ratings support higher sustained speeds but may compromise ride comfort.
- Prioritize performance – Those seeking better handling and braking may opt for lower-profile tires, while off-road aficionados may prefer more sidewall and tread. Identify your priorities.
- Consult experts – Discuss needs with knowledgeable tire and wheel professionals to dial in the optimal tire size. An expert can ensure proper fitment, speed calibration, and performance.
Following these tips when selecting a new tire size will help ensure you get ideal fit, function, and safety for your specific vehicle and driving demands.
Take the time to make the right choice.
