Tech: Information on Tire safety, inflation and speed ratings

We recently received a question about tire information, and what exactly all the numbers mean on the side of the tire.  Seeing how those 4 pieces of rubber are the only thing connecting you, the driver, to the road, we will provide a basic overview of tires and tire safety for passenger cars.  It will not be an end-all-say-all about tires, but it should provide you with enough information to help your decision making for a future set of tires.  Grab a cup of coffee if you need to stay awake as class is in session!  =) 

What makes a tire?  Simply put, a tire is constructed of mixture of rubber and silica compounds, with nylon or steel belts under the main tread to provide strength.  Tire construction varies with manufacturers, so each tire from a company will have a slightly different makeup compared to another.  The best tire to buy will depend on a number of factors:  your style of driving, weather conditions, price, etc. 

Passenger tires generally have sipes and grooves in them which aid in wet weather traction and channel water away from the tire to provide maximum contact with the road surface.  Brand new tires usually have at least 8/32" of tread.  To be legal for street use, a tire must have a minimum tread depth of 2/32".  Looking at your tire from above, you may see little "bars" which are embedded between the grooves.  These wear bars are indicators that your tire is approaching the limits of tire wear.  Once these bars are even with your tread, your tire is worn out.  To see if your tire is ok, take a penny and stick it in the groove of your tire with Lincoln's head facing down.  If most of his head is covered by the penny, your tire has at least 2/32" of tread left. 


However, be advised that that limit is the very minimum for depth.  When it rains, you still run a high risk of hydroplaning due to the fact that water cannot be compressed, and a tire with such low tread will not be able to effectively channel all the water away.  Hydroplaning will be felt as an immediate lack of steering feel as the tire essentially "floats" on the water under it.  Your best bet is to remain calm and try not stab the brakes or turn the steering wheel.  Ease off of the gas pedal until the hydroplaning is over.  Any sudden inputs with the steering or jabbing of the brakes may upset the balance of the car and send your vehicle into an uncontrollable slide.

Looking at the tire in the photo above, you will see some numbers which look like an elongated fraction such as 255/40/17.  Those numbers are in reference to the size of the tire.  The first number is with the width of the tire in millimeters.  255 means that from one side to the other, the tire is 255mm across.  The larger this number is, the wider the tire will be.

The second number is in reference to the height to width ratio also known as the aspect ratio.  The 40 means the height of the sidewall of the tire will be 40% of the width.  The higher the number, the larger the sidewall of the tire is.  High performance cars usually have a lower number to increase steering response and reduce tire deformation under cornering.  However, in most cases, a tire with a large sidewall rides more comfortably as it is able to absorb more of the shock and vibrations during driving.  Also, a low profile tire (small sidewall) will not provide as much protection to the wheel if a pothole is encountered. 

The last number is in relation to the diameter of the wheel the tire is intended to mount to.  In the case of our photo, it is made for 17" wheel. 

Many high performance tires have short sidewalls.

Now that you know a bit about the size of a tire, there are a couple more pieces of information you can get from your tire that may influence your purchase of your next set.  Typically after the tire size numbers, you will see another number such as 96Y, or 95H.  This number and alphabetical letter represent the load rating and speed rating of the tire.  The numerals show the tire's load rating, or how much a tire can support.  You can match it with the numbers in your owner's manual or call the manufacturer for the information.  Just as a note that load rating specification is not required by law so some manufacturers do not have any load information for their tires.  

The letter that follows the number is the speed rating of the tire.  The ratings show the maximum speed the tire is designed to go.  The categories are:


Q = 99 mph / 160 kph
R = 106 mph / 180 kph
S = 112 mph / 190 kph
U = 124 mph / 200 kph
V = 149 mph / 240 kph
Y = 186 mph / 300 kph
Z =  Above 149 mph / Above 240 kph

For tires that can exceed 186 mph, they are required to have Z in their designation by the D.O.T.

On many all season radial tires, you may see the letters M/S or M + S on them.  These letters indicate the tires have some mud and snow capabilities.  It does not mean that they can be a full on substitute of proper snow tires though!

Towards the inside of the tire, manufacturers often put more information regarding the construction of the tires, such as how many plies the tire has, and its composition.  There will also be a treadwear number such as 400, 300 or even 200.  These numbers will vary depending on the type of tire you buy.  The lower the number, the faster the tire will wear out, but lower treadwear tires generally have more dry grip.  Some high performance tires have ratings as low as 140!  These numbers are a bit arbitrary though, as there is no set standard in miles as to how the treadwear will be posted.  One manufacturer's 400 rating may be closer to another's 300 or 500 rating.  It's easier to use the treadwear rating within the manufacturer's own lineup of tires to compare longevity.  A 400 treadwear rating tire would be expected to last twice as long as a tire rated at 200.

Tires like the ones used on the Nissan GT-R will not last as long as the ones on a typical Nissan Versa.

Next, there is the word "Traction" followed by either: AA, A, B, or C.  The letters represent the ability of the tire to stop in on a wet surface.  The best is AA and the worst is C.  Summer tires or tires designed for maximum dry grip usually do not have the AA rating.  All season tires for the most part carry that designation.

The next word you will see is "Temperature" which refers to the tire's ability to resist heat.  The rating consists of A, B, or C.  This rating applies to a properly inflated tire that is not overloaded.  Under or over inflation, excessive loading or speeding will result in heat being built up in the tire and increases the risk for failure. 

Whew!  You've made it this far, and you have probably learned quite a bit about more about tires than you thought you could ever possibly want to know right?  Well, there are a couple more key things to realize about tires which could save you money either at the pump with better gas mileage or by extending the life of your tires reducing your purchases.

Before buying a new set of tires, take a look at their age.  It is recommended your tires not be over the age of 5-6 years as the rubber loses integrity.  All tires secrete out oils which keep them pliable, and as they lose the oils, they will begin to harden and crack or chunk.  The hardening increases the risk of a tire blowout.  You can determine the age of your tires by looking for the D.O.T. stamp on the tire.  Next to the D.O.T. stamp there will be another stamp with a couple letters and 4 digits.  These digits are the critical ones you need to pay attention to! 

Let's say for example you see:  3609 as your 4 digits.  This means your tire was manufactured in 36th week of the year 2009.  Another example would be 1211.  This would mean the tire was made in the 12th week of the year 2011. 

Most tires you buy will probably be about 1-2 years old already.  Make sure to check those numbers before you buy your tires and try to avoid any that are already 3 years or older.  If a shop refuses to let you check the tires, find another place.  It is also good to check the date on the tires you currently have.  If they are starting to get old, it might be wise to think about replacing them. 

Keep your tires properly inflated.  Your owners manual will have the recommended tire pressure listed.  Under inflating your tires increases rolling resistance and generates more heat.  This will hurt your wallet at the pump with reduced miles per gallon figures (mpg).  On the opposite end of the spectrum, you do not want to over inflate your tires as this reduces the amount of traction available for the tire as the contact patch is reduced.  You also risk tire failure since as the tire is driven it will heat up and naturally increase its air pressure.  If the air pressure exceeds the maximum safe limit by the tire manufacturer, there is a greater possibility the tire can blowout.

Proper inflation is critical to your tires.  Photo courtesy Frank Snider.

Under and over inflation will result in uneven tire wear.  Under inflated tires will wear on the outer edges of the tire as it essentially sags under load.  Over inflation will result in the center of the tire wearing out faster.  It is recommended to check the air pressure of your tires once a month.  In our experience, tires lose about 1 psi per month.

That wraps up our discussion on tire basics, and hopefully you have come away as a more informed consumer.  As always, if you have a question, feel free to ask!  Happy motoring and keep on driving!