How to Determine the Right Tire Size?

When you finally decide it's time to replace your tyres, you'll need to know exactly what size you should choose. Typically, this is found in the owner's manual, which can be found within the car's door jamb.

Learning the basics of tyre sizing will help you determine whether or not the tyres you currently have or are considering purchasing are the right size for your car. The series of numbers and letters on every tyre is a treasure trove of data you've probably never considered.

Below you'll find information and illustrations that will help you read tyre size from your tyres' walls if you're having trouble doing so. Many available tyre sizes may have different load limits, require wheels of a varying rim width or diameter, and demand different inflation pressure, therefore it's best to get advice from a tyre professional before making the switch.

Tire sizing is not always clear. Dimensions on the sidewall are presented in a variety of units, including millimetres and inches. Also, the way you drive can affect what size car, truck, or trailer is best for you.

The tyre size that came with your vehicle can be found in the owner's manual or on a sign attached to the inside of the driver's side door jam. The manufacturer of your car suggests this size.

Tire sidewalls typically feature a series of numbers and other indicators that might serve as a guide when shopping for replacement tyres. Next, consult a tyre expert for advice on what tyre sizes will work best for your vehicle and how you plan to use them.

There are just two rules of thumb to follow when deciding on the right tyre size.

• For starters, before replacing tyres, make sure they match the originals in size, load index, and speed rating.
• Second, it's important that the replacement tyres' size and weight capacity match the originals as closely as possible.

Finding the Right Tire Size

Select tyre size in strict agreement with vehicle documentation and manufacturer specifications. Distinct variations in riding height and speedometer readings can occur if the combined wheel and tyre diameter isn't properly matched.

Changing the wheel size on a vehicle that originally came with smaller wheels will affect the vehicle's ride height. This will result in a raised ride height for the vehicle's body. Aside from the inaccurate speedometer, max speed will improve little. Finally, the car's acceleration will decelerate.

Changing tyre sizes must be done in accordance with all laws and regulations, as well as the guidelines provided by the original equipment manufacturers of the vehicle, wheels, and tyres. The wheel must be able to turn freely in any direction, and the tire's load capacity must be kept in mind at all times.

Tires mounted on a car or truck must have a radial or bias-ply construction on the inside. Using a combination of bias-ply and radial tyres on a passenger vehicle or light truck is both illegal and unsafe. Only in case of extreme necessity may you utilise the spare tyre.

The same rules apply to selecting wheels or rims. Drivers are obligated to instal only the manufacturer-recommended wheels or rims on their vehicles.

To Change Less Than Four Tires

All four tyres should be replaced at once for maximum stability and handling. It is possible to replace less than all four tyres at once, but doing so requires adherence to a few standards based on tyre size.

In the event that you need to replace just one or two tyres, for instance, stick to the manufacturer-recommended tyre size, load index, and speed rating.

If you can only afford to replace two tyres, make sure they go on the back wheels. To put it simply, newer tyres provide superior traction, especially in the rain, and so lower hydroplaning chances.

It is not a good idea to replace just one tyre on a car because doing so can cause problems with the vehicle's suspension or gearbox and cause the remaining tyres to wear out prematurely. If only one tyre can be changed, the one with the deepest tread depth should be paired with an existing tyre and installed on the rear axle.

Sizing in Metric

Tires are typically either P-Metric or Euro-Metric on most automobiles, SUVs, and light pickups (1/2 tonne and smaller). Tires that fall under the "P-metric" category, will have a "P" before the size designation (e.g., "P225/70R16 97H") on the sidewall. Euro-Metric tyre sizes will simply be written as a series of digits, like so: 225/70R16 98H. The European Tyre and Rim Technical Organization has defined the term "Euro-Metric" to describe a certain category of "passenger car" tyres. Cars, minivans, SUVs, and other light-duty pickup trucks are the primary users of P-Metric and Euro-Metric tyre sizes.

Heavy-duty light vehicles and vans, such as 3/4 tonne and bigger pickup trucks and vans, sometimes have a different type of size classification on their placard than passenger cars and SUVs. Standard sizes in this group are LT-Metric and Euro-Metric Commercial (aka C-type). While both systems are metric and follow the same basic structure as P-Metric and Euro-Metric, the dimensions of commercial vehicles are slightly different. Example tyre sizes for LT-Metric tyres include: LT245/75R17 119/116R Load Range E. Also note the two load index values and the Load Range. If you want to know more, head on over to the Load Index bit.

The standardised term "LT-Metric" to refer to tyres designed for "light trucks." Commercial or C-Type Euro-Metric tyre sizes look very similar to passenger Euro-Metric tyre sizes (23/65R16C 121/119R), with the addition of a "C" right after the rim size. There are two load index numbers for C-type tyres. The European Tyre and Rim Technical Organization has defined the term "Euro-Metric Commercial," sometimes known as "C-Type," to refer to tyres designed for light trucks. Light truck tyres are made for vehicles that can carry a lot of weight, and are typically required by the manufacturer when the vehicle's load capacity is higher than the manufacturer's standard.

Temporary Spares are another category of tyres that use Metric measurements. All of their names begin with the letter "T." The "ST" designation indicates that the size is suitable for usage solely on a trailer.

The size designations on tyres don't change whether you're looking at P-Metric, Euro-Metric, LT-Metric, Euro-Metric Commercial, T, or ST tyres.

Width

Your tyres should have a width of P225/70R16 91S, where P225/70 refers to the width in millimetres and R16 to the height of the tyre.

The width of a tyre is always measured from sidewall to sidewall. The "P225" designation indicates that the tire's nominal width is 225 millimetres, making it appropriate for use on passenger vehicles.

Scale Factor

A tire's aspect ratio, or the height of its profile in relation to its width, is represented by the number that comes after the slash. For example, a set of tyres with the dimensions P225/70R16 91S would have an aspect ratio of 91%. The aspect ratio of a tyre is determined by dividing its height above the rim by its width during production. A tyre with an aspect ratio of 70 means that its height is 70 percent of its width.

In most cases, tyres with a lower aspect ratio, such as those in the 60 series, are preferable to those with a higher ratio, such as those in the 75 series, for improving a vehicle's handling performance. However, ride harshness is often a compromise.

Construction

The P225/70R16 91S designation follows the aspect ratio and denotes the tire's internal construction, which plays a role in ensuring the tire's stability.

On a tire's sidewall, you might see one of two constructions:

• Letter "R" for Radial
• Bias Ply or Diagonal

The radial tyres are by far the norm, hence the letter "R" is typically included in the tyre size classification. The term "radial construction" refers to the fact that the inner ply cords of the tyre run from one bead to the other in a path that is roughly perpendicular to the direction of rotation. Run-flat tyres (designated with a "RF") and tyres rated for speeds above "V" (designated with a "ZR") can also be found on the market.

Sizing the Rims

The following digit represents a code for the rim's diameter in inches; here is where the tyre will be placed. For a 16-inch wheel, a tyre marked P225/70R16 91S, for instance, would be the proper size.

Indicator of Demand Load

We'll do our best to clear up any confusion you may have about the load index, which has a lot of qualifiers. The number that follows the rim size in the tire's model number is the load index, which gives us the maximum load the tyre can carry when fully inflated in pounds: P225/70R16 91S.

The number does not tell us how many pounds the tyre can handle, therefore we refer to it as the load "index" instead. This value, however, is in accordance with a particular index entry for maximum load. The digits in the load index, which start at 1, denote weight carrying capacities ranging from 99 to 7385 lbs.

However, passenger tyres can be either Standard Load or Extra Load. No further markings will be included on a standard load tyre, whereas the letters XL will be placed after the size and load index on an extra load tyre.

Passenger car tyres, such as P-Metric and Euro-Type, will have a single load index number, while LT-Metric and Euro-Type Commercial (C-Type) tyres will have two numbers separated by a slash. If the tyre is used for only one purpose, the first number is the load index; if it is used for both, the second number is the load index. Tires designed for passenger vehicles cannot be utilised for commercial purposes. The Load Range of light truck tyres is also denoted by a letter, such as Load Range E. Although Load Range is an outdated word, it is still extensively used in the business, so you may hear your tyre dealer speak to it. Load index numbers are still the most reliable indicator of whether or not a tyre is appropriate.

Load index is often misunderstood, but an important detail is that the load index numbers between standards organisations (P-Metric vs Euro-Metric) are not necessarily on the same scale. This means that two tyres in the two systems with the same load index number could have different maximum load capacities. Since this is the case, it is crucial to check the load index to ensure the stated capacity is accurate.

Ranking of Speeds

Just as the load index number indicates the maximum weight the tyre can carry, the speed rating letter indicates the maximum speed the tyre can handle as measured in a controlled laboratory environment.

For instance, a "S" rated tyre can handle speeds of up to 112 mph, whereas a "R" rated tyre can only handle speeds of up to 106 mph. Keep in mind that this isn't a suggested average speed. But, obviously, you should never break the law when it comes to speeding on roads.

If you want to keep your vehicle's top speed, you need to be sure that the replacement tyres you get have a comparable or greater speed rating than the OE tyres. The top speed of a vehicle with tyres of varying speed ratings is determined by the slowest tyre.

Measurements for Flotation

If you're looking for off-road tyres for a full size pickup or SUV, there's one more size classification you should be aware of. The numbers in a Flotation size are significantly different from those in a Metric size. Flotation-sized tyres are functionally equivalent to LT-Metric tyres, with a few key distinctions. The first issue is that they can't be utilised for multiple purposes, and the second is that a standard tire's weight capacity may vary from that of its LT-Metric equivalent.

Learn What Size Tires Will Work for Your Vehicle

By increasing the tire's width by one size and decreasing the aspect ratio (the height of the tire's sidewall in relation to its width), the tire's overall dimensions can be kept roughly the same. A P195/60R15, for instance, can carry up to 87 pounds of weight and has an overall diameter of 24.2. The load index remains the same at 87 even if you upgrade to a bigger tyre with a lower aspect ratio, the P205/55R15, and the overall diameter remains the same at 23.9 inches. When interpreting a tire's sidewall, the central number, 60 or 55, represents the aspect ratio.

When deciding on a new set of wheels and tyres, online tyre size calculators can come in handy. There are online tyre size calculators that list the sizes that will fit a given vehicle model. Such is the case with the tyre selector tool from consumer reports, which also provides a list of additional tyre sizes for the specified vehicle type and emphasises tested tyres that are compatible with them.

Is It Time for New Rims or Tires?

Use a tyre size calculator to quickly determine if a particular tyre size will fit your vehicle, SUV, sports car, light truck, or crossover.

But keep in mind that's just a rough guess. It's crucial that you don't exceed the permissible dimensions of your car. Tires that aren't the right size can make your ride rougher and noisier, create a pull to be felt in the steering wheel, and even lower your vehicle's ground clearance, especially on slopes.

Consult a tyre professional before deciding to change your vehicle's tyre size. See if the tyres and wheels you're considering will work with your car by checking the dimensions of the vehicle's suspension, transmission, and body. Find out how variables like RPM, tyre speed, load index, and speed rating may influence your ride comfort and vehicle performance.

To Which Tire Type Should You Change?

Retail sites often provide a complete catalogue of tyres in your size. However, you'll typically have to look more into the matter to match the speed rating. Here is a list that can be used to determine what kind of tyres you have.

• There are two available speed ratings for all-season tyres: S and T. These are typically installed on family sedans and crossover SUVs because of their high mileage and reliable performance in any weather.
• Many modern vehicles, particularly those aimed at enthusiasts or those with modified wheels, come equipped with performance all-season tyres with either a H or V speed rating. Performance tyres often have greater cornering grip than all-season tyres rated S or T, but they may not last as long.
  ZR, W, and Y speed ratings are common among ultra-high performance all-season and summer tyres for sports vehicles and performance sedans.
• To learn the specifics of what makes a set of tyres all-season or summer-specific, you may need to visit the tyre maker's website. The absence of the M&S (Mud & Snow) marking on the sidewall of a summer tyre is one way to distinguish it from its winter counterpart.
• For the purposes of towing and hauling with light-duty trucks and SUVs, all-season and all-terrain truck tyres are often available in big sizes. Tires designed specifically for use off-road typically have a more aggressive tread pattern. One helpful hint is to look for the letters "A/T" or "All Terrain" in the model name of the tyres you're interested in.
• The sidewall of winter/snow tyres typically features a mountain and snowflake emblem. In addition, there are more slits, or sipes, in the tread, making it look busier than all-season tyres. To get the best stopping and handling performance, buy a set of four winter tyres.

Which Characteristics of a Tire Are Most Important to You?

We found that while people's cars are still relatively new, they are more likely to purchase a straight replacement tyre. However, as time passes, buyers are more likely to trade up to a newer model for either better performance or a lower price.

Tires with maximum tread life and all-weather grip are rated highly, so if you're in the market for a new set, be sure to peruse our detailed ratings before making a purchase.

Tire buyers can find a wealth of information and resources online from a variety of manufacturers. Furthermore, websites such as TireRack.com and Discountiredirect.com provide convenient car selections. Some websites, like RightTurn.com, even go the extra mile by incorporating installation at a nearby merchant in the final price and providing customers with individualised product options.

Most of the internet stores we've shopped from have met our expectations. Good prices can also be found at many chain stores, discount warehouses, and independently operated tyre shops. You should compare prices after you've settled on a brand of tyres.

Exactly What Size Are You Looking For?

To begin, you should look up the suggested tyre dimensions in your owner's manual or on the plaque attached to the inside of the driver's side doorjamb. You can expect to see something like "P215/60R16 94T" on the label.

The initial designation, "P215/60R16," indicates the tire's dimensions (such as width and diameter). The 94 then denotes the load index, or the maximum load that can be carried by a single tyre. Last but not least, T represents the tire's maximum speed in relation to the load index.

Tire sizes should be matched as closely as possible, however load index and speed ratings can be increased.