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Tire Sidewall Information in Detail

How to read a tire

As an example here is a common automobile tire size: 205/55R16 89H

205 – The section width of a tire.  The distance between the two outermost parts of the tire section (width), expressed in millimetres.  This is not to be confused with the tread width.  The tread (or the part of the tire in contact with the ground) is usually much narrower than the section width and can vary from one tire to another.

55 – The profile.  The height of a tire sidewall, measured from the mounting surface of the wheel to the top of the tread.  This figure is expressed as a percentage of the section width.  (In this example, 55% of 205mm, or 112.75mm.) 

R – The construction of the tire, in this case radial.  Radial refers to the direction of the ‘frame’ of the tire.  Radial is at 90 degrees to the direction of rotation.  A vast majority of tires today are radial in construction.  The other type of construction available is diagonal or bias type.  They are sometimes found in trailer, agricultural or OTR tires, but not in modern automotive tires.

16 – The diameter of the wheel (or rim), in inches.  It is measured from the tire counting surface, so its very difficult to calculate on a wheel from the outside.

89 – A number representing the maximum load a tire can operate, at its maximum operating temperature.  The actual load can be found in load tables, or in detail on the sidewall of the tire.

H – A letter representing the maximum speed a tire can maintain for a prolonged period of time.  A speed-rating table can convert the letter to the actual speed rating.

 

Other possible markings 

 

XL or RF Extra Load or Reinforced

Tires built with reinforcements in the construction to accommodate higher air pressures.  The added air pressure could be for handling characteristics, in the case of performance tire sizes or added loads, in the case of light truck tire sizes.

 

Prefix  P, LT, ST 

P – P-Metric sized (example P205/55R16 89H) is a North American standard for tire sizes.  If there is nothing before the tire size (example 205/55R16 89H) the tire size is Euro-Metric, or the European standard.  In terms of size and performance, these tires are virtually identical except for a slight difference in the way loads are calculated at various air pressures.  (In the above example, these 2 tires have the same load rating at their maximum pressure, but vary slightly at each operating pressure up to that maximum load/pressure.)  These tire sizes are considered equivalent and can be used interchangeably (ideally 4 on a vehicle, but at the very least 2 by 2, always on the same axel).

LT – Light Truck designates a tire built for additional load ratings.  These tires are constructed to accommodate higher air pressures, thereby the can carry higher loads.  For more information.  Sometimes on Euro-Metric sizes, this added load designation is a ‘C’ after the tire size, for Cargo.

ST – Service Trailer designates a ‘Service Trailer’ tire, or a tire specifically designed to de used on trailers.  Since they are not designed for forces such as acceleration or lateral changes of direction, they should never be used on motorised vehicles.

T – Temporary Spares have this designation.  These are the ‘mini-spares’ or ‘space saver’ spare tires.  These are designed to operate at lower speeds and for restricted distances.

 

Floatation sizes

Some tire sizes are expressed in ‘Flotation’ size.  Usually found on larger tires for pick up trucks and SUVs for off road use.  The tire sizes are expressed in Imperial formats and the LT designation is at the end of the tire size.

Example  33x11.5R16LT 

The 33 represents the overall diameter of the tire, rounded up to the nearest inch (usually one half in less).  The 11.5 is the section width of the tire in inches.  The 16 is the same as standard size tires, that is the diameter of the wheel in inches.  There usually exists a ‘regular’ tire size equivalent (in this case LT305/70R16) to the flotation size, but it doesn’t sound as cool for serious off roaders.

 

UTGQ

A standardized test, created in the mid 70’s to help customers chose a tire based on three criteria:  Treadwaer, traction and temperature.  Although they could be used many years ago, today, they are no longer relevant and not used by tire professionals.  For more information see here

M+S – Indicates that the tire is ‘All-season’.  Not to be confused with approved for severe winter conditions. For more information see here.

Mountain/Snowflake – Designation for tires approved for use in severe winter conditions. For more information see here.

DOT -  The identification code of a tire, as approved by the Department of transportation.  The code represents a myriad of information including what plant the tire was manufactured, the tire model and size, internal information for the tire manufacturer and the date (week and year) the tire was manufactured. For more information see here.

MAX. LOAD – Information specific to the maximum operating load of the tire, including the maximum operating pressure.  Please note that the correct operating pressure for your vehicle is indicated on a placard in the vehicle, this information moulded into the sidewall of the tire is the maximum operating pressure.

Construction.  Tires have the number and material of the plies used in their construction.  This can help determine how the tires are built and for what purpose.  Be aware that this has absolutely nothing to do with the ply rating or load characteristics of the tire. For more information see here.

 

Speed Ratings

The speed rating is the maximum speed at which a tire was designed to operate.  It’s a rigorous test involving high speeds and high loads for specified periods of time.  The maximum speed is represented by a letter, moulded on to the sidewall of a tire, usually grouped with the load rating. See the chart. 

Generally speaking, the higher the speed rating, the higher the performance characteristics of the tire and (usually) the lower the durability.  The speed numbers may seem extreme.  For example, a standard touring tire has a speed rating of ‘S’ or a maximum sustained speed of 180 km/h.  Many people see this and think ‘I’ll never drive anywhere near that speed’.  What you must consider is that it is a performance rating rather than a speed rating.  That is to say that a tire with a speed rating of 180 km/h and another with a speed rating of 270 km/h will not have the same performance, even at 110 km/h. 

When choosing a replacement tire, you must consider the speed rating your vehicle was originally equipped with as well as your driving style and what characteristics you want from your tires. Your vehicle’s suspension and steering was designed for a specific performance type, by modifying your tire’s speed rating could adversely affect the handling characteristics of your vehicle.

 

Load Ratings

The load rating is a 2 or 3 digit number moulded into the sidewall of a tire, usually grouped with the speed rating, to identify the maximum load a tire can carry at its maximum air pressure. The maximum load, as well as the maximum operating air pressure, is also moulded into the sidewall of the tire in small writing closer to the tire rim.  Please note that this tire pressure is the maximum operating pressure for the tire.  The recommended tire pressure for your vehicle is found in the placard in the door.

Since dynamic vehicle loads (applied to tires) are calculated using the load rating, make sure to replace your tires with the same load rating as the original equipment tires.  If you modify the load rating, make sure to adjust the air pressure accordingly (using the proper load table information).

 

P vs. LT

Pick up trucks and SUVs come in two sizes: big and bigger.  When driving a larger vehicle, designed for higher loads, that vehicle will probably come equipped with LT designated tires.  When a tire has the letters ‘LT’ in the tire size, they are designated as ‘Light Truck-metric’ and are designed to support additional loads.  Essentially, the tires are constructed to operate at higher air pressures.  For general automotive use, there are 3 load ranges: C, D and E.  They refer to the maximum operating tire pressure (C – 50 psi, D – 65psi, E – 80 psi). 

When replacing LT tires, make sure you replace them with the same load range as the original equipment.  If you replace your tires with lower load ratings, you risk operating your vehicle in overload situations that could lead to a blow out.  Increasing the load rating could also be harmful, as the vehicle (wheels, valves, axels, etc) must be strong enough to handle the higher air pressures and associated loads.

 

UTQG

The Uniform Tire Quality Grade Standards is a test that was developed by the Department of Transportation (DOT) many, many, many years ago.  It was developed to help consumers chose the appropriate tire for their needs, but unfortunately, it is no longer relevant in today’s market.

The first thing to understand is that the testing is done by tire manufacturers to distinguish their own products.  That means that you cannot compare ratings from one manufacturer to another.

  • Treadwear:  Vehicles are driven in a convoy, on a preset course in West Texas for 7,200 miles.  The tires being tested are compared to a ‘standard’ tire and the resulting wear is then compared to the ‘standard’ tire.  The ‘standard’ tire is given a score of so if the tire being evaluated lasts twice as long, it receives a score 200.  Here’s the catch: The tests are conducted under very optimal conditions.  Tire pressure, vehicle alignment and wheel rotations are frequently checked and adjusted.  It is impossible to account for weather differences.  Since the tire is usually not very worn after the test, tire manufacturers must extrapolate the data to determine the final result.  Tire manufacturers may also reduce the resulting number in order for the tire to fit better into its line up.  What was originally designed to be a tool for consumers is now more of a tool for marketing departments. 
  • Traction: A test to determine the coefficient of friction (traction) on wet surfaces as a tire is pulled along various types of road (asphalt and concrete, etc) and the wheel is locked up.  The depending on how high the friction, the tires are given a grade of A, B or C.  In 1997, a new rating of AA was added, since tires were far exceeding the maximum standard. Since the test is performed as static test (the tire is not moving) this offers little or no information to a consumer about lateral performance such as cornering.  In addition it provides no information on dry traction or efficiency of water evacuation at high speeds. 
  • TemperatureThis test is performed in a laboratory by applying extreme loads to the tire at high speeds to see how well a tire can dissipate heat and resist blowing out from an overload.  They are graded with an A, B or C depending on how well they perform, or at what speeds they can tolerate.  As stated, this test is no longer relevant to general consumers and is not used by tire professionals.  There is work being done on a revised test that may include more trustworthy data.  The NHTSB is attempting to create a better, more consumer friendly indicator of fuel efficiency, wet traction and durability.

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