How to Determine the Right Tire Size?

Once you have determined it's time to buy tires, you'll need to know what size tires are correct for your vehicle. This information is usually inside your car's doorjamb, in your owner's manual.

To ensure your current tire or a replacement tire you may be looking at matches your vehicle's requirements, it will be good to understand how tire sizing works. You may have never paid attention to the string of numbers and letters on every tire, but it's a gold mine of information.

If you're unsure how to read tire measurements from your tire walls, the information and graphics below will tell you how to read tire size and understand and interpret it. If you decide you want to substitute a new size or tire type, consult an authorized tire retailer who can expertly advise you because many optional tire sizes may have different load capacities and require wheels of a different rim width or diameter and different inflation pressure.

Tire size can be confusing. For example, some numbers on the sidewall are listed in millimetres, while others are inches. Plus, the right size for your car, truck, or trailer can differ depending on where and how you drive.

You can see your original equipment tire size in your owner's manual or on the placard generally located on the driver's side door jam. This is the sizing recommended by the vehicle manufacturer.

If you're interested in switching out your tires for a different look or performance, the numbers and other indicators on your existing tires' sidewall is a good place to start. Next, have a tire professional help you determine a tire size range that will fit your vehicle and driving needs.

When considering tire size, there are two simple rules to follow.

  • Firstly, the replacement tires should be the same size, load index, and speed rating specification as the vehicle or tire manufacturer recommended.
  • Secondly, the replacement tires must never be smaller or have load-carrying capacity than the original specification.

Not many people know how to tell tire size, check out the guide on how to do it.

Choosing the Correct Tire Size

Make your tire size selection 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 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 recommendations of the vehicle, wheel, and tire manufacturers. At the very least, the wheel must have complete freedom of circular motion, 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 are 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.

Replacing Fewer Than Four Tires

For optimal safety and performance, replace all four tires at the same time. While it is possible to switch out less than four tires simultaneously, 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 a much better grip, particularly on wet roads, and reduce the risks of hydroplaning.

Replacing only a single tire is not advisable, as it can impact vehicle suspension or transmission and produce excessive wear on the tyre tread. But if replacing only one tire is unavoidable, pair the single replacement tire with the deepest tread depth, then fit both to the rear axle.

Metric Sizing

Most passenger cars, SUVs, and light pickups (1/2 ton and smaller) will come with either P-Metric or Euro-Metric tires. For P-Metric tires, you'll see the letter "P" before the number sequence begins: P225/70R16 97H. P-metric is a designation standardized by the Tire and Rim Association for a "passenger car" tire type. There will be no initial letter for Euro-Metric before the number sequence begins: 225/70R16 98H. Euro-Metric is a designation standardized by the European Tyre and Rim Technical Organization for a "passenger car" tire type. Both P-Metric and Euro-Metric size tires are designed to primarily be used on passenger vehicles, including cars, minivans, SUVs, and other light-duty pickup trucks.

If your vehicle is an SUV, Pickup truck or van, you might see a different type of size designation on your placard specific for heavy-duty light trucks and vans, especially common on ¾ ton and larger pickup trucks and vans. There are two common size types in this category, LT-Metric and Euro-Metric Commercial (aka C-type). Both size types are metric and use the same structure as P-Metric and Euro-Metric but have some different characters in the size that differentiate them from their passenger car cousins. For example, LT-Metric tires will have the letters "LT" before the size number sequence: LT245/75R17 119/116R Load Range E. Notice that there are two load index numbers and a Load Range. See the section on Load Index for more info.  

LT-Metric is a designation standardized by the Tire and Rim Association for a "light truck" type tire. Euro-Metric Commercial or C-Type tires will look very similar to a passenger Euro-Metric size except that there will be a "C" right after the rim size: 23/65R16C 121/119R. Notice that the C-type tires also have two load index numbers. Euro-Metric Commercial or C-Type is a designation standardized by the European Tyre and Rim Technical Organization for a light truck type tire. Light truck tires are designed to be used on vehicles capable of carrying heavy cargo and are usually only specified by a vehicle manufacturer on vehicles exceeding a certain load capacity.

Other types of tires that fall into the Metric sizing type are Temporary Spares. They start with "T". If you see a size that starts with "ST," that means "special trailer" and is only for use on a trailer.

Regardless of whether you are looking at a P-Metric, Euro-Metric, LT-Metric, Euro-Metric Commercial, T, or ST tire, the numbers in size mean the same thing.

Width

The first number to appear in your tire size information is the width, in millimetres, of the correct tires for your vehicle: P225/70R16 91S.

Tire width always refers to the measurement from one sidewall to another. Thus, a tire with the measurement "P225" is for a passenger vehicle and has a nominal width of 225 millimetres.

Aspect Ratio

After the slash mark, the next number you see is for the tire's aspect ratio, which essentially tells you how tall your tire's profile is: P225/70R16 91S. Aspect ratios are delivered in percentages. Tire makers calculate the aspect ratio by dividing a tire's height off the rim by its width. For example, if a tire has an aspect ratio of 70, its height is 70% of its width.

Lower aspect ratio tires, such as a 60 series, generally offer vehicle handling performance advantages over higher aspect ratio tires, such as a 75 series. Still, a typical trade-off can be ride harshness.

Construction

After the aspect ratio comes to a letter indicating the type of internal construction maintaining your tire's stability: P225/70R16 91S.

There are two types of construction that you may see on the sidewall of a tire:

  • R – Radial
  • D or "B" or "-"– Diagonal or Bias Ply

Radial tires are the most common tires in the United States today; thus, "R" will usually be shown in the tire size designation. Radial construction means the tire's internal ply cords are oriented in a radial direction, from one bead over to the other, essentially perpendicular to the direction of rotation. You may also occasionally see RF indicating a run-flat tire or ZR indicating a tire with a speed rating higher than V.

Rim Diameter

The next number is the diameter code, in inches, of the rim onto which the tire can be mounted. For example, a tire with the P225/70R16 91S would fit a rim with a 16-inch diameter.

Load Index

The load index can be confusing because there are so many different caveats, but we will try to explain everything here.

The next figure after the rim size in the sequence is your tire's load index, which tells us how much weight, in pounds, the tire can support when fully inflated: P225/70R16 91S

We call it the load "index" because the number doesn't tell us the precise number of pounds the tire can carry, at least not by itself. However, the number does correspond to a specific load capacity listed in an index. Beginning with 1 and ending with 150, numbers in the load index represent 99 to 7385 lbs carrying capacities.

There are two types of load types for passenger tires, though, Standard Load and Extra Load. If a tire is Standard Load, there will be no markings indicating it, but if it is Extra Load, the letters XL will appear after the size and load index.

Standard Load Euro-Metric: 215/55R17 94V

Extra Load Euro-Metric: 215/55R17 98V XL

Passenger car tires like P-Metric and Euro-Metric will only have one load index number, where LT-Metric and Euro-Metric Commercial (C-Type) will have two numbers separated by a slash. The first number is the load index if the tire is used in a single application, the second is the load index if the tire is used in a dual application. Passenger type tires cannot be used in a dual application. Light truck tires will also have a Load Range that is indicated by a letter, such as Load Range E. Load Range is an older term that is still commonly used in the industry so that you may hear your tire dealer reference it. Still, the load index numbers are the best way to ensure you have the proper tire.

One important but often misunderstood facet about load index is that the load index numbers between standards organizations (P-Metric vs Euro-Metric) are not necessarily on the same scale, which means that two tires in the two different systems with the same load index number could have different maximum load capacities. This is why it's important to look at the load index number and verify the actual load capacity.

Speed Rating

The final figure in a tire size sequence is the speed rating, indicated by a letter: P225/70R16 91S. Just as your load index number corresponds to a specific load, your speed rating letter corresponds to a particular speed capability based on a standardized laboratory test.

For example, a tire with speed rating "S" is rated for up to 112 mph, while a tire rated "R" is up to 106 mph. Remember that this isn't a recommended cruising speed. But, of course, you should always follow legal speed limits on roadways.

Replacement tires must have the same or higher speed rating as the vehicle's Original Equipment to maintain vehicle speed capability. If a vehicle has tires with different speed ratings, the speed rating of the "slowest" tire dictates the vehicle's top speed.

Flotation Sizing

There is one last sizing type that you should know about, especially if you are in the market for off-road tires for a light truck or SUV. It's called a Flotation size, and the numbers in this sizing format are very different from the Metric formats. Flotation sized tires are similar to LT-Metric tires in the application except for a few important points. Number one, they cannot be used in dual applications, and number two, an equivalent tire may have a different load capacity than its LT-Metric counterpart.

How to Identify a Compatible Tire Size

Changing one size up in tire width and one size down in aspect ratio (the ratio of the tire sidewall height to width) will maintain approximate tire size. For example, a P195/60R15 has an overall diameter of 24.2 inches and a load capacity of 87. Move up to a wider tire with a lower aspect ratio, P205/55R15, and the overall diameter is similar, at 23.9 inches, and the tire has the same load index of 87. The aspect ratio is expressed in the centre number, 60 and 55, respectively, in reading the tire sidewall.

Online tire size calculators can be helpful when choosing alternate tire and wheel sizes.

Some tire size calculators give the size tire available for a specific make and model car. For example, consumer Reports' tire selector tool can do that, plus it lists other sizes available for that car model and highlights compatible tested tires.  

Buying New Wheels or Changing Your Tire Size?

A tire size calculator is a quick way to see whether the tire size you're considering will likely fit your car, SUV, sports car, light truck or crossover.

But remember that is only an estimate. It's important to stay within the sizing tolerances of your vehicle. Tires that are the wrong size could cause some pull in the steering wheel, rub against the suspension or body of your vehicle, reduce clearance on hills, or result in a stiffer or noisier ride.

If you're considering mounting a different tire size on your vehicle, check with a tire expert. Find out whether the tires and wheels you have your eye on are the right fit for your vehicle's suspension, gearing, and bodywork. And ask how any differences in revolutions per mile, tire speed, load index, and speed rating will affect your ride quality and vehicle performance.

Faqs

What Type of Tire Do You Need?

Many retail websites will give you a listing of all tires available in your size. But in many cases, you'll need to dig deeper to match the speed rating. The list below can help ID your tire type.

  • All-season tires come in S- and T-speed ratings. Known for good all-weather grip and long mileage, these are commonly fit for mainstream cars and SUVs.
  • Performance all-season tires come in H- and V-speed ratings on many newer cars, especially those with enthusiast appeal or upgraded wheels. They tend to have better cornering grip than S- and T-speed rated all-season tires, but performance tires may not wear as long.
  • Ultra-high-performance all-season and summer tires typically come in ZR-, W-, and Y-speed ratings for sports cars and performance sedans. Differentiating between all-season and summer tires can be challenging and require going to a manufacturer's website to find out the details. One clue to tell them apart is that a summer tire would not have an M&S (Mud & Snow) designation on the sidewall.
  • All-season and all-terrain truck tires naturally come in large sizes and are designed for light-duty pickups and SUVs' hauling and towing duties. All-terrain tires generally have a more aggressive tread pattern to aid off-road traction. A tip is that many all-terrain tires will have "A/T" or "All Terrain" right in the model name.
  • Winter/snow tires are easily identified by a mountain and snowflake symbol displayed on the tire's sidewall. Plus, the tread looks busier than all-season tires with lots of slits, known as sipes. When shopping, be sure to buy winter tires in sets of four to optimize braking and handling.

What Are Your Priorities in Selecting a Tire?

Our research shows that people often choose a direct replacement tire when the car is still relatively new. But as the car ages, consumers become more inclined to switch to another model based on performance or price.

If you're looking to make a switch, be sure to check our extensive tire ratings, especially if you're seeking a model with maximum tread life and all-weather grip.

Many manufacturers have websites to help consumers choose the right tire. In addition, online retailers like TireRack.com and Discountiredirect.com have simple-to-use vehicle selectors. Some sites like RightTurn.com go further, giving consumers personalized choices and an all-in-one price, including installation at a local retailer.

We have had success with most online retailers we buy from. But many local tire dealers, big-box stores, and tire-company-owned retailers can offer good deals, too. So shop around once you have made your tire choice.  

What Size Do You Need?

First, consult your owner's manual or the placard on the driver's side doorjamb to find the recommended tire measurements. The label will look something like this: P215/60R16 94T.

The first part of the label—P215/60R16—refers to the tire's various size measurements, such as width and diameter. Next, the 94 indicates the load index, which is how much weight each tire can support. Finally, the T is the speed rating, the tire's maximum speed about the load index.

You should match the tire's size measurements, but you have some flexibility to go higher with the load index and speed rating.

Frequently Asked Questions About Tire Size

Many retail websites will give you a listing of all tires available in your size. But in many cases, you'll need to dig deeper to match the speed rating. The list below can help ID your tire type.

  • All-season tires come in S- and T-speed ratings. Known for good all-weather grip and long mileage, these are commonly fit for mainstream cars and SUVs.
  • Performance all-season tires come in H- and V-speed ratings on many newer cars, especially those with enthusiast appeal or upgraded wheels. They tend to have better cornering grip than S- and T-speed rated all-season tires, but performance tires may not wear as long.
  • Ultra-high-performance all-season and summer tires typically come in ZR-, W-, and Y-speed ratings for sports cars and performance sedans. Differentiating between all-season and summer tires can be challenging and require going to a manufacturer's website to find out the details. One clue to tell them apart is that a summer tire would not have an M&S (Mud & Snow) designation on the sidewall.
  • All-season and all-terrain truck tires naturally come in large sizes and are designed for light-duty pickups and SUVs' hauling and towing duties. All-terrain tires generally have a more aggressive tread pattern to aid off-road traction. A tip is that many all-terrain tires will have "A/T" or "All Terrain" right in the model name.
  • Winter/snow tires are easily identified by a mountain and snowflake symbol displayed on the tire's sidewall. Plus, the tread looks busier than all-season tires with lots of slits, known as sipes. When shopping, be sure to buy winter tires in sets of four to optimize braking and handling.

Our research shows that people often choose a direct replacement tire when the car is still relatively new. But as the car ages, consumers become more inclined to switch to another model based on performance or price.

If you're looking to make a switch, be sure to check our extensive tire ratings, especially if you're seeking a model with maximum tread life and all-weather grip.

Many manufacturers have websites to help consumers choose the right tire. In addition, online retailers like TireRack.com and Discountiredirect.com have simple-to-use vehicle selectors. Some sites like RightTurn.com go further, giving consumers personalized choices and an all-in-one price, including installation at a local retailer.

We have had success with most online retailers we buy from. But many local tire dealers, big-box stores, and tire-company-owned retailers can offer good deals, too. So shop around once you have made your tire choice.  

First, consult your owner's manual or the placard on the driver's side doorjamb to find the recommended tire measurements. The label will look something like this: P215/60R16 94T.

The first part of the label—P215/60R16—refers to the tire's various size measurements, such as width and diameter. Next, the 94 indicates the load index, which is how much weight each tire can support. Finally, the T is the speed rating, the tire's maximum speed about the load index.

You should match the tire's size measurements, but you have some flexibility to go higher with the load index and speed rating.

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