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Everything You Ever Wanted To Know About Tires But Was Afraid To Ask.
Most passenger cars, SUVs and light pickups (1/2 ton and smaller) will come with tires that are either P-Metric or Euro-Metric. 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. For Euro-Metric there will be no preceding letter 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, which can include 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 that is 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 so 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. 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 the size mean the same thing.
The first number to appear in your tire size information is the width, in millimeters, 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 millimeters.
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 number 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 you may hear your tire dealer reference it but 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. Meaning that two tires in the two different systems that have the same load index number could have different maximum load capacities. This is why it’s important to not only look at the load index number but also verify the actual load capacity.
The final figure in a tire size sequence is the speed rating, which is 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. 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, it is the speed rating of the “slowest” tire that dictates the vehicle top speed.
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 application except for a few important points. Number one, they cannot be used in dual applications and number two, an equivalent size tire may have different load capacity than its LT-Metric counterpart.
The first number in the Flotation tire size is the overall diameter in inches. Pretty straight forward.
intelligent buying decision. However, the ratings are based upon test results achieved under special conditions. This means it’s possible to misinterpret the comparative data as it relates to your individual driving habits, conditions, etc. You should still rely on your service or tire professional for assistance.
Quality grading designates the comparative performance levels of a tire based on government-specified tests but commissioned by the individual tire manufacturers. All tire manufacturers are required to grade regular and all-season passenger tires in three categories:
The treadwear grade is a comparative rating based on the wear rate of the tire when tested under controlled conditions on a specified government test course for 6,000 miles (9,600 km). For example, a tire graded 150 would wear one and a half times as well on the government course as a tire graded 100. However actual tire performance depends on driving habits, road characteristics, service practices, and other factors that can influence the outcome.
Traction Grades AA, A, B and C
The traction grades from highest to lowest are AA (the highest), A, B and C. They represent how well tires stop on wet pavement as measured under controlled conditions on specified government test surfaces of asphalt and concrete. C-rated tires will have the lowest traction performance.
WARNING: THE TRACTION GRADE ASSIGNED IS BASED ON A WET BRAKING (STRAIGHT AHEAD) TRACTION TEST AND DOES NOT INCLUDE CORNERING (TURNING) TRACTION.
Temperature Grades A, B and C
The temperature grades A, B, and C repreSizes in Metric
Most passenger cars, SUVs, and light pickups (1/2 tonne and under) will have either P-Metric or Euro-Metric tyres as standard equipment. Before the number sequence starts, you’ll notice the letter “P” for P-Metric tyres: P225/70R16 97H. The Tire and Rim Association has established the term “passenger car” tyre as “P-metric.” There won’t be a letter before the number sequence starts in Euro-Metric. 225/70R16 98H. The European Tyre and Rim Technical Organization has established the term “passenger car” tyre as “Euro-Metric.” Both P-Metric and Euro-Metric size tyres are intended to be used largely on passenger cars, vans, SUVs, and other light-duty pickup trucks.
You may notice a different type of size designation on your placard if your vehicle is an SUV, pickup truck, or van. This form of size identification is frequent on pickup trucks and vans that are 34 tonne and greater, and it is specialised for heavy duty light trucks and vans. This category includes the typical size types LT-Metric and Euro-Metric Commercial (aka C-type). Both size categories are metric and hence share the same structure as P-Metric and Euro-Metric, but they differ from their passenger vehicle
Because there are so many distinct caveats, load index can be a difficult topic to understand, but we’ll do our best to do so here.
Your tire’s load index, which is listed after the rim size in the sequence, indicates how much weight, in pounds, the tyre can support when completely inflated: P225/70R16 91S
We refer to it as the load “index” because the number, at least by itself, does not specify the specific amount of pounds the tyre can support. The figure does, however, match a particular load capacity mentioned in an index. The numbers in the load index denote carrying capacities ranging from 99 to 7385 lbs, and they go from 1 to 150.
For passenger tyres, however, there are two load types: Standard Load and Extra Load. There won’t be any markings on a tyre if it is Standard Load, but if it is Extra Load, the letters XL will show after the size and load index.
215/55R17 94V Standard Load Euro-Metric
215/55R17 98V XL Euro-Metric Extra Load
P-Metric and Euro-Metric passenger vehicle tyres have a single load index number, but LT-Metric and Euro-Metric Commercial (C-Type) have two digits that are separated by a slash. If the tyre is used in a single application, the first number is the load index; if it is used in two applications, the second number is the load index. Passenger-type tyres cannot be utilised for two different purposes. A light truck tire’s load range will also be denoted by a letter, for example, Load Range E. However, the load index numbers are the best way to be sure you have the right tyre. Load Range is an older term that is still frequently used in the business.
The fact that the load index numbers of standards organisations (P-Metric vs. Euro-Metric) are not always on the same scale is a crucial but sometimes misunderstood aspect of load index. So, even when two tyres in two distinct systems have the same load index number, they could have different maximum load capacities. This is why it’s crucial to confirm the real load capacity in addition to the load index number.
The speed rating, denoted by a letter and the last number in a tyre size sequence, is P225/70R16 91S. Your speed rating letter refers to a certain speed capability based on a conventional laboratory test, just as your load index number corresponds to a particular load.
For instance, a tyre with a speed rating of “S” is capable of 112 mph, whereas a tyre with a speed rating of “R” is capable of 106 mph. Keep in mind that this is not a suggested cruising speed. Of course, you should always adhere to the posted speed limits on public roads.
To preserve a vehicle’s ability to travel at a certain speed, replacement tyres must have a speed rating equal to or greater than the Original Equipment. If a vehicle has tyres with various speed ratings, the top speed of the vehicle is determined by the speed rating of the “slowest” tyre.
One more sizing kind exists, which you should be aware of if you’re searching for off-road tyres for a light vehicle or SUV. The figures in this sizing system, which goes by the name of a flotation size, are substantially dissimilar from those in metric standards. With a few key exceptions, flotation sized tyres are applied similarly to LT-Metric tyres.
The treadwear grade is a comparison ranking based on the tire’s wear rate during testing over a 6,000-mile period in controlled settings on a designated government test course (9,600 km). On the government course, a tyre graded 150, for instance, would wear 1.5 times as much as a tyre marked 100. However, real tyre performance depends on driving patterns, the nature of the route, maintenance procedures, and other variables that may have an impact.
Grades AA, A, B, and C of Traction
The traction grades are AA (the highest), A, B, and C, in order from highest to lowest. They show how well tyres stop on wet pavement when tested on designated government test surfaces made of asphalt and concrete under regulated conditions. The least effective tyres for traction are those with a C rating.
CORNERING (TURNING) TRACTION IS NOT INCLUDED IN THE TRACTION GRADE, WHICH IS BASED ON A WET BRAKING (STRAIGHT AHEAD) TRACTION TEST.
A, B, and C graded temperatures
When evaluated under controlled circumstances on a specific indoor laboratory test wheel, the temperature grades A, B, and C describe the tire’s resistance to the creation of heat and its capacity to dissipate heat. Long-term high temperatures can shorten tyre life and accelerate material degradation, while extreme temperatures might induce unexpected tyre failure. According to Federal Motor Vehicle Safety Standard No. 109, all passenger automobile tyres must perform at a level that corresponds to the grade C. Grades A and B indicate performance levels above the minimal legal requirements on the laboratory test wheel.
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. If a tire has an aspect ratio of 70, it means the tire’s 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, but a typical trade off can be ride harshness.
After the aspect ratio comes a letter that indicates 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 on the road 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 that is a speed rating higher than V.
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 can be a confusing subject 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 carrying capacities of 99 to 7385 lbs.
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
The second number is the section width (sidewall to sidewall) measurement in inches. Again, fairly simple.
After the section width comes a letter that indicates the type of internal construction: 33X12.50R17LT 120Q.
This is the same as is found in the metric sizing systems.
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 on the road 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.
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 33X12.50R17LT 120Q would fit a rim with a 17-inch diameter.
The letters LT will be after the Rim Diameter indicating that this tire type is intended for Light Truck vehicles similar to the LT-Metric and Euro-Metric Commercial (C-Type) tires.
Load Index and Speed Rating
Load Index and Speed Rating have the same meaning and format as the tires using the metric sizing system. Note that since flotation tires cannot be used in a dual application there will be only one load index number instead of two.
Uniform Tire Quality Grading
Another group of stamping on certain types of tires is the Uniform Tire Quality Grading or UTQG. This grading and stamping is required for passenger car tires (i.e. P-metric and Euro-metric) in the all season and summer categories. Dedicated winter tires, Light Truck (LT-Metric, Euro-Metric Commercial, Flotation) and Motorcycle tires are excluded from this requirement.
Quality grading is designed to make the tire purchase decision easier for you. Ideally, the system is intended to provide simple, comparative data so you can make an
relatives in a few key ways. The initials “LT” will appear before the size number sequence on LT-Metric tyres: LT245/75R17 119/116R E-load range. You’ll note that there are two load index numbers, as well as a load range; for further information, refer to the section on load index. The Tire and Rim Association has standardised the term “light truck” type tyre as “LT-Metric.” Unless there is a “C” immediately before the rim size, commercial or C-type Euro-Metric tyres will look quite similar to passenger Euro-Metric sizes: 23/65R16C 121/119R. Take note of the two load index numbers on the C-type tyres as well. The European Tyre and Rim Technical Organization has defined the term “Euro-Metric Commercial,” sometimes known as “C-Type,” for a light truck tyre. Light truck tyres are made for cars that can tow high loads, and a vehicle manufacturer will often only specify them for those that can tow loads greater than a particular amount.
Temporary Spares, which begin with the letter “T,” are another kind of tyres that fit into the Metric sizing type. The size “ST” stands for “special trailer” and is solely intended for use on trailers.
The numbers in the size mean the same thing whether you are looking at a P-Metric, Euro-Metric, LT-Metric, Euro-Metric Commercial, T, or ST tyre.
The first figure in your tyre size information is the width of the proper tyres for your car, expressed in millimetres: P225/70R16 91S.
The dimension from one sidewall to the next is always referred to as the tyre width. Therefore, a tyre marked “P225” is for a passenger car and has a nominal width of 225 millimetres.
The following number you see after the slash represents the aspect ratio of the tyre, which effectively indicates how tall the profile of your tyre is: P225/70R16 91S. Deliveries of aspect ratios are done in percentages. The aspect ratio is calculated by dividing the tire’s height off the rim by its width by tyre manufacturers. When a tire’s aspect ratio is 70, its height is equal to its width in proportion.
A frequent trade-off can include ride harshness. Lower aspect ratio tyres, such a 60 series, typically offer vehicle handling performance advantages than higher aspect ratio tyres, like a 75 series.
The letter P225/70R16 91S, which denotes the kind of internal construction keeping your tyre stable, appears after the aspect ratio.
There are two different construction styles that you could notice on a tire’s sidewall:
D or “B” or “-” – Diagonal or Bias R – Radial Ply
Due to the prevalence of radial tyres on American roads today, “R” will typically appear in the tyre size designation. In a tyre with radial construction, the internal ply cords are arranged from one bead to the next in a radial pattern that is virtually perpendicular to the direction of rotation. Occasionally, you might also notice RF standing for a run-flat tyre or ZR for a tyre with a speed rating higher than V.
Size of the rim
The rim’s diameter code, expressed in inches, is the following number, which is where the tyre can be placed. A tyre with the dimensions P225/70R16 91S, for instance, would fit a rim with a 16-inch diameter.
In addition, an equivalent size tyre may have a different load capacity than its LT-Metric counterpart, hence they cannot be utilised in dual applications.
The overall diameter in inches is indicated by the first number in the Flotation tyre size. Pretty simple to understand.
The section width, measured from sidewall to sidewall, is indicated by the second digit. Again, not too difficult.
The section width is followed by the letter 33X12.50R17LT 120Q, which designates the interior construction style.
The metric sizing systems use the same terminology for this.
There are two different construction styles that you could notice on a tire’s sidewall:
R = Radial D, B = Diagonal, or – = Bias Ply
Due to the prevalence of radial tyres on American roads today, “R” will typically appear in the tyre size designation. In a tyre with radial construction, the internal ply cords are arranged from one bead to the next in a radial pattern that is virtually perpendicular to the direction of rotation.
Size of the rim
The rim’s diameter code, expressed in inches, is the following number, which is where the tyre can be placed. A tyre with the dimensions 33X12.50R17LT 120Q, for instance, would fit a rim with a 17-inch diameter.
Similar to the LT-Metric and Euro-Metric Commercial (C-Type) tyres, the initials LT will be located after the Rim Diameter to indicate that this tyre type is designed for Light Truck vehicles.
Speed Rating and Load Index
The meaning and format of the Load Index and Speed Rating are identical to those of the tyres with the metric size standard. There will only be one load index number rather than two because flotation tyres cannot be used in a dual application.
Grading of Tire Quality uniformly
The Uniform Tire Quality Grading, or UTQG, is a different set of stamps that are used on particular types of tyres. For passenger car tyres (i.e., P-metric and Euro-metric) in the all-season and summer categories, this grading and stamping is necessary. This criterion does not apply to motorcycle tyres, light truck (LT-Metric, Euro-Metric Commercial, Flotation), or dedicated winter tyres.
The purpose of quality grading is to simplify your selection to buy tyres. The system’s ultimate goal is to give you straightforward, comparable information so you can choose your purchase wisely. The scores, however, are based on test findings obtained under unique circumstances. This implies that the comparable statistics may not accurately reflect your unique driving patterns, environmental factors, etc. You should continue to seek assistance from your service or tyre specialist.
Quality grading refers to the relative performance levels of a tyre based on tests that were commissioned by the various tyre manufacturers but were prescribed by the government. Regular and all-season passenger tyres must be classified into one of three categories by all tyre manufacturers:
WARNING: THE TEMPERATURE GRADE WAS DETERMINED FOR A TIRE THAT WAS ADEQUATELY INFLATED AND NOT OVERLOADED. Underinflation, excessive loading, and excessive speed can all contribute to heat buildup and potential tyre failure.
Quality Scores for DOT
In addition to these ratings, all passenger automobile tyres must adhere to other government regulations.sent the tire’s resistance to the generation of heat and its ability to dissipate heat when tested under controlled conditions on a specified indoor laboratory test wheel. Sustained high temperature can cause the tire’s material to degenerate and reduce tire life, and excessive temperature can lead to sudden tire failure. The grade C corresponds to a performance level all passenger car tires must meet under the Federal Motor Vehicle Safety Standard No. 109. Grades A and B represent higher levels of performance on the laboratory test wheel than the minimum required by law.
WARNING: THE TEMPERATURE GRADE IS ESTABLISHED FOR A TIRE THAT IS PROPERLY INFLATED AND NOT OVERLOADED. EXCESSIVE SPEED, UNDER INFLATION, OR EXCESSIVE LOADING, EITHER SEPARATELY OR IN COMBINATION, CAN CAUSE HEAT BUILDUP AND POSSIBLE TIRE FAILURE.
DOT Quality Grades
All passenger car tires must conform to other federal requirements in addition to these grades.