How to Read a Tire Sidewall
Understanding tire sizes, numbers, and codes.
The sidewall of a tire can be a perplexing place. Aside from the brand and model, the seemingly random assortment of letters and numbers can leave the layperson thinking they should leave it to a tire shop to figure out. The truth is that it does not take a CIA cryptographer to become a tire expert. At the end of this article, you will know most of what there is to know. So what do all those letters and numbers on a tire sidewall mean? Let's dive in. For the purposes of the article our real life example tire will be a 255/35ZR19 Michelin Pilot Sport 4S as found on a 2022 Mercedes-Benz CLA45.
Quick Reference Links
Size Designation or Service Type
The first component of a tire size tells us how to read the rest of the components. That letter is known as the Service Type or Size Designation.
|Code||Tire Service Type Prefix|
|_||No prefix. Euro-metric for use on passenger vehicles. Sometimes referred to as "Hard-metric". This is the most common type of tire for cars, crossovers, SUVs, minivans, and smaller pickup trucks.|
|P||P-metric for use on passenger vehicles.|
|LT||Light Truck-metric for use on full-size pickups, heavy-duty trucks, and other vehicles capable of carrying or towing heavy loads.|
|ST||Special Trailer service for use only on trailers.|
|T||Temporary service for use as a spare tire.|
Sometimes a tire size will have a trailing Size Designation or Service Type. These are the following postfix Service Type and Size Designations.
|Code||Tire Service Type Postfix|
|C||Commercial service. Used with Euro-metric sizes to indicate use on vehicles capable of carrying or towing heavy loads.|
|HL||High Load service. When HL is used the size will have a trailing XL. This signifies a higher load capacity versus an extra load tire in the same dimensions.|
|LT||Light Truck "flotation" or "high-flotation" size designation for use on full-size pickups, heavy-duty trucks, and other vehicles capable of carrying or towing heavy loads. Sizes are expressed in inches.|
It is recommended that your replacement tires use the same Service Type. If your vehicle came equipped with LT service type tires replace them with LT service type tires. P-metric and Euro-metric tires can be used interchangably. Their difference is slight and technical in nature. As you will note on the sidewall of our 255/35ZR19 tire, the P or LT as is commonly found on other tire sizes is omitted. This indicates that the tire is a Euro-metric tire. The vast majority of tires will be expressed in *-metric sizes and will be the focus of the rest of this article. Please see our upcoming Flotation tire sizing guide for LT tires.
Tire Size Code
We now know our Michelin PS4S are of the Euro-metric variety. Most consumer tires utilize either the P-metric, Euro-metric, or Light Truck-metric sizing nomenclature which all share the same following characteristics.
Section Width or "Treadwidth"
Sometimes referred to as the treadwidth, the 255 in our example 255/35ZR19 tire size is the section width of the tire. The section width is the cross section of the tire from inner to outer sidewall. This means that in theory the lower the specified section width, the skinnier the tire. (Boo!) The higher the specified section width, the wider the tire! (YAY!) A common misconception is that the 255 represents the treadwidth, or the amount of rubber making contact with the road. This is not the case!
The section width indicated in a tire size is not exact and is more useful as a variable in calculating the overall diameter of a tire than the true section width or the true treadwidth of a tire. 255 millimeters converted to inches is 10.0". However, the specifications for a 255/35ZR19 Michelin Pilot Sport 4S are a 10.2" section width and 8.5" treadwidth, while the equivalent 255/35ZR19 Yokohama Advan A052 has a 10.2" section width and 9.5" treadwidth. Both Michelin and Yokohama specify these respective tires as having a 26" overall diameter and 799 revolutions per mile, but the Yokohama Advan A052 has 1" more treadwidth and thus 1" more contact with the ground. Clearly not all 255/35/19 are created equal.
The next number in the size sequence Aspect Ratio, sometimes referred to as the "Profile". The sidewall aspect ratio is the height of the sidewall expressed as a percentage of the Section Width. You may hear someone refer to a 35 series or 35 profile tire. This number is not comparable from tire to tire and not very useful without the accompanying section width of the tire. For instance a 315/35 would have a sidewall height of 110.25 while our example 255/35ZR19 would have a 89.25mm sidewall height. While both are "35 series" tires, their sidewall heights have more than a 1cm height difference.
If a letter follows the Aspect Ratio this denotes the internal construction of the tire. Nearly all tires sold today feature a steel radial construction and the internal construction designation R is sometimes omitted for a "/".
|R||R represents a Radial construction tire.|
|ZR||ZR represents a Radial construction tire rated for speeds over 149mph. If rated for over 186mph it must have a ZR designation.|
|D||D represents a Diagonal (bias-ply) construction tire.|
|RF||RF represents a Run Flat construction tire.|
There are two common variations of the internal construction designation that you may see used on a tire. ZR is a legacy designation that means the tire has been designed to exceed speeds of 149mph. Many modern cars can exceed that speed, and the tire's Service Description provides more precise information via the Speed Rating. It is still used by many manufacturers today to denote a performance oriented tire. The RF designation is used to denote a "Run Flat" tire. Run Flat tires feature reinforced sidewalls that will allow you to continue driving a short distance if the tire is punctured.
The final number in the size sequence is the Wheel Diameter. If you take a ruler to the face of your wheel you will see that the diameter of the face does not match the tire's indicated wheel diameter. The Wheel Diameter is the distance in inches from the wheel's internal bead seat to its opposite bead seat in the vertical direction.
Why is the Section Width expressed in metric units while the Wheel Diameter is expressed in standard units? The answer either has to do with the history and evolution of global tire size specs or a vast conspiracy to make you better at math.
Overall Tire Diameter
With the final number in the size sequence we can calculate the overall diameter of our 255/35ZR19 tire.
( Section Width * ( Aspect Ratio / 100 ) ) * 2 + Wheel Diameter
( 255 * ( 35 / 100 ) ) * 2 + (18 * 25.4) = 635.7
( ( 255 * ( 35 / 100 ) ) * 2 * 0.0393701 ) + 18 = 25.03
To make matters even more confusing, it is important to note this calculation only represents a theoretical overall tire diameter. The actual physical size is determined by factors such as the rim width, tread depth, or the tire manufacturer's actual production dimensions. As we noted above, according to TireRack our Michelin Pilot Sport 4S in 255/35ZR19 has a 26" overall diameter as does the 255/35ZR19 Yokohama ADVAN A052 while our calculation brings us to a 25" overall diameter. If you are working with very tight fitment tolerances contact the tire manufacturer for measurements of the exact tire you are considering.
Not to be confused with the Service Type or Size Designation, the tire Service Description describes the load and speed capabilities of a tire.
The Tire Load Index is a numerical value which maps to the maximum load carrying capacity of the tire at maximum load pressure. This Load Index is not the same as the Load Rating, which we will cover shortly. Typical load index values range from about 80 to 120, but the full index ranges from 0 (45kg) to 150 (3350kg). The numerical value maps to the following load carrying abilities where a higher number indicates a higher load carrying capacity. When viewed in a chart you can see the load indices are a metrically imposed measurement.
|Load Index||Lbs||Kgs||Load Index||Lbs||Kgs|
If you are looking to replace your tires, it is fine to switch to a new tire that has a higher Load Index than your factory or OEM tires. This simply means the new tires can safely carry more weight. Your vehicle came equipped with a tire that was matched to the car so consider the original tires as having the minimum Load Index you should be replacing them with. Always choose a tire that has a greater or equivalent Load Index.
Our example Michelins have a service description of (96Y) XL. This means that at maximum air pressure the tires can support a maximum load of 6,260lb or 2,840kg (4 tires x 710kg). These specific Michelin's are installed on a 2022 CLA45 with a maximum gross vehicle weight rating of 4,795lbs or 2,175kg, leaving more than enough room for safety.
With LT or ST tires you may see two load indexes. LT and ST tires are sometimes mounted with paired wheels. The first number in a dual rated tire is the Load Index when used individually and the second number is the Load Index when used in tandem.
The Speed Rating is an alphanumeric value which represents the maximum speed the tire is certified to safely run at. The Speed Rating starts as low as A1 which are rated to 5km/h or 3mph. Most tires will have a speed rating of L or above, except for the H speed rating which is curiously placed between U and V. O is omitted for obvious reasons. When viewed in a chart you can see the Speed Ratings follow a metric measurement pattern.
|Speed Rating||mph||km/h||Speed Rating||mph||km/h|
You may have heard the term "Z-Rated" tires and you may have seen their legacy when you see ZR in a tire's internal construction designation. At the time, Z speed rating tires were rated above 149mph or 240km/h. It was thought a higher speed rating would not be required. Cars slowly evolved to have rather insane capabilities, and thus W and Y ratings were introduced. All tires capable of over 186mph or 300km/h must have a ZR construction designation in the tire size.
There is one more symbol to consider. What do the parentheses around the service description on our 255/35ZR19 (96Y) XL Michelin PS4S tires mean? When the service description is surrounded with parentheses the top speed of the tire has been tested to greater than 186mph / 300km/h. The manufacturers have done this for us so there is no need to test ourselves.
The final component typically quoted with basic tire sizes is the Load Range or Ply Rating. The Load Index has a maximum load pressure at which the maximum load is certified for. That maximum load pressure is determined by the Load Range. Thought of in another light, the Load Range describes the ply rating of the tire and whether or not it is constructed with additional reinforcement.
Extra Load or XL tires add extra plies and steel belts to the edge of the tires for extra strength. This results in stiffer sidewalls and lowers the likelihood of tire damage caused by potholes and other road hazards. This reinforced construction allows Extra Load tires to stand up to higher loads than a Standard Load tire in the same size. The penalty for this reinforcement is in weight. Though negligible, a comparable Standard Load tire will be lighter than an Extra Load tire and provide better rotational efficiency. At the end of the day, the tread design and proper inflation will determine the tire's efficiency more than the Load Range.
For P-metric tires there are three load ranges with their respective maximum load pressures. Euro-metric tires also have three load ranges with ever so slightly different maximum load pressures. This is the primary difference between P-metric and Euro-metric tires. For a given Load Rating, a Euro-metric tire will have a slightly higher theoretical load carrying ability than a P-metric tire.
|Size Designation||Load Range||Abbreviation||Maximum Load Pressure|
|P-metric||Light Load||LL||35psi / 240kpa|
|P-metric||Standard Load||SL or Unmarked||35psi / 240kpa|
|P-metric||Extra Load||XL||41psi / 280kpa|
|Euro-metric||Standard Load||SL or Unmarked||36psi / 250kpa|
|Euro-metric||Extra Load or "Reinforced"||XL or RF||42psi / 290kpa|
|Euro-metric||High Load Extra Load||HL and XL||42psi / 290kpa|
If your inner OCD is also annoyed by the single psi difference in P-metric vs Euro-metric load pressures, rest assured that future P-metric sizes will use the same 36psi or 42psi maximum load pressures.
As with the Load Index, when changing tires we want to match or better the Load Range of the replacement tire. Extra Load tires can replace Standard Load tires. Extra Load tires should be replaced with Extra Load tires. If you run a tire in high-performance applications or with very thin sidewalls, consider the Extra Load tires an extra level of safety and shop for a tire with a XL rating.
UTQG: Uniform Tire Quality Grade
The UTQG, or Uniform Tire Quality Grade, standards were developed by the U.S. DOT's NHTSA to provide standardized information regarding Treadware, Traction, and Temperature which ratings you will see on the sidewall of a tire. As "uniform" as these standards were intended, we will get in to some details that make them less uniform as the government would have hoped. In fact, the National Highway Traffic Safety Administration does not test the tires as they might crash test a vehicle. These tests and ratings are performed by the tire manufacturers or a NHTSA contractor.
Our example Michelin Pilot Sport 4S has a UTQG of:
TREADWEAR 300 TRACTION AA TEMPERATURE A
The first UTQG component you will encounter on a tire is the Treadwear. The UTQG Treadwear rating, sometimes abbreviated tw, is an indicator of how long the tire is expected to last. The treadwear testing process (WARNING: Nerdy 36 page document) dates back to June of 1979 and is performed over a 400 mile test loop for 7,200 miles at the UTQG Test Facility centered at Goodfellow Air Force Base and local roads around San Angelo, Texas. The tire is run alongside a reference tire known as a Course Monitoring Tire or CMT. If the tire under test is expected to last just as long as the CMT it receives a UTQG Treadwear of 100. Treadwear grades are expressed in multiples of 20. If it is expected to last twice as long it receives a UTQG Treadwear of 200. Five times as long? You guessed it: UTQG Treadwear of 500.
Here is where the Uniform Tire Quality Grade becomes less uniform. Tire manufacturers do not have to rate their tires at the exact UTQG Treadwear. They are allowed to under report treadwear, which means they can rate a tire at 200 treadwear even though it tested at 300 treadwear. Note, manufacturers are not permitted to over report treadwear. Phew. The test loop is only 7,200 miles so the actual tire life beyond that point is also more of an estimate from the tire manufacturer based on test data. Due to the way this data is under reported and extrapolated by the manufacturers or their consultants, it is only useful to compare treadwear within a manufacturer's product lines. You should not expect one tire manufacturer's 500tw to wear at the same rate as another tire manufacturer's.
Going back to our example, you should expect our 300 treadwear Michelin PS4S to have 3 times or more of the tread life of a 100 treadwear rated Michelin.
The second UTQG component is Traction. The UTQG Traction rating is an indicator of the coefficient of friction (μ), or grip, the tire under test has in a straight line on a wet surface. Note this is not a test of dry grip or traction, overall performance, or resistance to hydroplaining. A 57-page NHTSA document describes the Tire Traction Testing Procedures. The TL;DR is that the tire is mounted and tested alongside a control tire that is used to test the coefficient of friction on both an asphalt and concrete skid pad. Both the control and tires under test are inflated to 24psi and mounted to a trailer traveling at 40mph. The brakes on the trailer are locked at which point the coefficient of friction is measured as the tire skids across the wet surface. This coefficient of friction then determines the UTQG Traction grading.
|UTQG Traction Grade||Asphalt||Concrete||Notes|
|AA||>0.54μ||>0.38μ||15% of current tires.|
|A||>0.47μ||>0.35μ||77% of current tires.|
|B||>0.38μ||>0.26μ||7% of current tires.|
|C||<0.38μ||<0.26μ||Less than 1% of current tires.|
These procedures are, like the UTQG Treadwear procedures, from 1979 so do not account for modern advances such as anti-lock brakes. All tires need to run through this procedure to be DOT approved nonetheless. It bears repeating that the UTQG Traction grade is not an indicator of dry traction or performance. Our example Michelin PS4S has an AA UTQG Traction rating which indicates it will still have quite a bit of friction in a straight wet skid.
The final UTQG component is Temperature. The UTQG Temperature rating is an indicator of how well the tire under test resists generating heat. High temperatures at speed can cause a reduction in tire life or even tire failure. The tire is spun on a drum at increasing rotations per minute (rpm) with a load of 88 percent of the tire's maximum load rating. The test starts at250 rpm for 2 hours, then 375rpm for 30 minutes, and then in increasing 25rpm increments until the tire fails or the tire has run at 575rpm without failing. Failure is defined is deformation or visual evidence of degradation such as chunking, bead separation, or broken cords.
Grade C is the minimum performance level for a tire to meet federal regulations and it indicates that the tire failed prior to the 500rpm test stage. Grade B indicates the tire successfully passed the 500rpm test but failed before the 575rpm test. Finally, Grade A incidates that the tire successfully completed the entire test up to 575rpm. What does all of that translate to in the real world? The respective vehicle speeds of the tires as mapped to the test are as follows:
|UTQG Temperature Grade||Vehicle Speed mph||Vehicle Speed km/h|
|A||> 115mph||> 185km/h|
|B||Between 100 to 115mph||Between 161 to 185km/h|
|C||Between 85 to 100mph||Between 137 to 161km/h|
Our example Michelin PS4S has a UTQG Temperature rating of A. Feel free to mount them up for your next long road trip through the desert.
Special Service Markings
M+S: Mud and Snow Marking
There are two special markings to look for on your sidewall to indicate "all-weather" capabilities. The M+S stands for Mud and Snow and indicates the tire has a block tread pattern designed with more traction capabilities. The M+S designation dates back to the 1970s to differentiate these new tire designs from simple ribbed treads that were prevalent at the time. When a tire is marked as M+S it meets the geometric requirements of the Rubber Manufacturers Association (RMA) guidelines for some mud and snow use.
Though a tire may be marketed as "all-season" or "all-weather", without the M+S designation it has not been certified for packed snow and ice. Note that the M+S, MS, M&S, or M/S markings do not indicate that this is a dedicated winter tire. It simply means the tread block design matches those that are designed for extra traction in inclement weather. An M+S marking is not a guarantee of all-weather performance and is not a replacement for a dedicated snow and winter tire.
3PMSF: Three Peak Mountain Snowflake Symbol
The latest in "all-weather" markings is the 3PMSF symbol. Near the turn of the century, North American tire manufacturers decided it was important to differentiate truly winter capable tires from those marketed as "all-seasons" with a compatible tread block. The 3PMSF symbol is literally a three peak mountain with a snowflake and indicates the tire has met a minimum level of acceleration traction on medium packed snow.
This is where this industry symbol fell short of denoting a truly all-weather capable tire. The test only verifies acceleration traction and does not require a minimum amount of braking or cornering performance. It also does not cover ice traction. While tires carrying a 3PMSF marking can be expected to have inclement weather performance beyond that of an M+S tire, they are not a replacement for a dedicated snow and winter tire.
OE Tire Markings
Originating from Porsche's N-Spec designation, many tires today now carry their own Original Equipment (OE) marking. The OE Tire marking identifies tires that are approved for use by the vehicle manufacturer. This marking is meant to make it easier to identify an exact replacement for your original tire. Some of the known OE Tire designations follow in the chart below.
|Make||Model||OE Tire Marking|
|Alfa Romeo||AR, ARR|
|Aston Martin||A4A, AMS, AM2, AM8, AM9, AMR, AMX|
|Audi||A, AO, AOE, RO1, RO2|
|Bentley||B, B1, BC, BL|
|Ferrari||F, K1, K2|
|GM||TPC SPEC, TPC SPEC 1156 M+S, TPC SPEC 1156|
|Mercedes||MO, MO1, MOE, MO-S, MO-V|
|Porsche||All Pre-2019||N0, N1, N2, etc.|
|911||NA0, NA1, NA2, etc.|
|Boxster / Cayman||NB0, NB1, NB2, etc.|
|Cayenne||NC0, NC1, NC2, etc.|
|Macan||NE0, NE1, NE2, etc.|
|Panamera||ND0, ND1, ND2, etc.|
|Taycan||NF0, NF1, NF2, etc.|
Many tire buyers will simply default to the exact tire that came with the car. Buyer beware: while some OE tires may have slight differences, the same brand and model of tire can typically be found without the OE marking at a lower cost. In the case of our 255/35ZR19 (96Y) XL Michelin PS4S, it carries Mercedes-Benz's MO1 designation but the same model and size can be found for other OEs.
|OE Tire Designation||Size||Load Rating||Price|
|GOE Genesis||255/35R19||96Y XL||$296.99|
|MO1 Mercedes||255/35ZR19||(96Y) XL||$296.99|
|Star BMW||255/35ZR19||(96Y) XL||$370.52|
|Star BMW||255/35R19||96Y XL||$310.99|
The Michelin Pilot Sport 4S in a 255/35/19 can be found in six different configurations, with varying load and speed ratings, and 3 different OE designations. There are even two "Star BMW" rated tires in the variations, each with their own price. To make matters even more confusing, the Genesis and Mercedes OE designated tires are less expensive than the (96Y) XL tire that has no OE designation. The moral of the story is that you may be able to find the same tire compound with the same ratings as your OE designated tire for less money. Manufacturers like Porsche gather up tires three times a year for six weeks to put them through the ringer so that they truly earn their Porsche N-rating. While the OE tire marking is not purely marketing, keep in mind that you absolutely do not have to replace your tires with the same brand and model tires your vehicle came with and the rating process costs both the tire company and the vehicle manufacturer marketing dollars that they must make back.
DOT Tire Identification Numbers
Any tire used on public roadways in the United States must carry Department of Transportation (DOT) Tire Identification Numbers (TIN). The DOT Tire Identification Numbers identify the manufacturing location, a tire size and type code, and the date the tire was manufactured.
Our example Michelin Pilot Sport 4S has a DOT TIN of:
DOT F3 N4 026X 3121
The DOT TIN always starts with DOT, followed by an eight to 13 alphanumeric code that represents the tire factory, the tire size code, the tire type code, and the date code. Our tire was made at the Manufacture Française des Pneumatiques Michelin in Roanne, France represented by F3. N4 is the size code for our tire which is specific to Michelins. 026X represents the type code which, again, is specific to Michelins.
Finally, the 3121 represents the date code of the tire. The tire date code is a four-digit number representing the two-digit week of manufacture and the two-digit year of manufacture. This means our tire was made the 31st week, beginning August 8th, of 2021. This date code is for tires manufactured after the year 2000. Prior to 2000, date codes contained three-digits containing the week of manufacture and the year of the decade.
Un-fun fact! Rubber rots. How old is too old for tires? It is recommended you replace your tires when they are six years old, regardless of the remaining treadlife. If you are using a 90s or earlier tire in this millenium please stop looking at the date code and replace your tires!
Basic Construction Materials
If you are looking to
bore impress someone at a dinner party, be sure to take note of the construction materials marked on the sidewall of your tire. A tire's basic construction materials are molded in to the sidewall, showing the material at the center of the tread and the widest point of the sidewall.
Our example Michelin Pilot Sport 4S presents its basic construction materials as:
TREAD PLIES: 1 POLYESTER + 2 STEEL + 1 POLYAMIDE SIDEWALL PLY: 1 POLYESTER
Our Michelin Pilot Sport 4S has an Extra Load rating which indicates it likely has steel cords to reinforce the sidewall structure. These sorts of materials will not be present at the widest point of the sidewall so they are not included in the basic construction materials marking.
There are a couple of extra markings you should know about if you find yourself mounting a tire or checking your local shop's work.
Tires with an asymmetric tread pattern will have an inner and outer tread pattern. The sidewall closest to the outer tread pattern will be marked OUTSIDE. The inner sidewall will, of course, be marked INSIDE. Asymmetric tread designs will generally feature wider tread blocks on the outside for increased cornering grip. When mounted inversely, a tire will exhibit suboptimal wear, water channeling, and traction characteristics.
Tires with a directional tread pattern will be marked with an arrow and most of the time the word rotation with an integrated arrow. At times the arrow will be disguised as part of the visual design of the sidewall branding, so you may need to look closely.
A directional tire is mounted correctly when the arrow is facing the direction of forward rotation. This is counter-clockwise when mounted on the left side of the vehicle and clockwise when mounted on the right side of the vehicle. Note that when referring to vehicle sides, the sides are in reference to the driver's perspective in the vehicle while facing forward. When mounted in the opposite direction of the intended design, a tire will exhibit suboptimal wear, water channeling, and traction characteristics.
You did it. You got to the end. Amazing. Have a burning question you want answered in a guide? Email us at firstname.lastname@example.org.
All prices as of original publishing. ↩︎