The ABC’s of Tires

When Should I Replace My Tires?

According to most states’ laws, tires are legally worn out when they have worn down to 2/32″ of remaining tread depth. To help warn drivers that their tires have reached that point, tires sold in North America are required to have molded indicators called “wear bars” across their tread pattern from their outside shoulder to inside shoulder. Wear bars are designed to visually connect the elements of the tire’s tread pattern and warn drivers when their tires no longer meet minimum tread depth requirements.

However, as a tire wears it is important to realize that while its dry traction and handling will improve…its ability to perform in rain and snow will diminish. At 2/32″ of remaining tread depth, resistance to hydroplaning in the rain at highway speeds has been significantly reduced and traction in heavy snow has been virtually eliminated.

If rain and wet roads are a concern, you should consider replacing your tires when they reach approximately 4/32″ of remaining tread depth. Since water can’t be compressed, you need enough tread depth to allow it to escape through the tire’s grooves. If the water can’t escape fast enough your vehicle’s tires will be forced to hydroplane (actually float) on top of the water, losing traction.

If snow covered roads are of concern, you should consider replacing your tires when they reach approximately 6/32″ of remaining tread depth to maintain good mobility. The reason that you need more tread depth in snow is because your tires need to compress the snow in their grooves and release it as they roll. If there isn’t enough tread depth, the “bites” of snow your tires can take on each revolution will be so small that your traction will be reduced. Because tread depth is an important element for snow traction, winter tires start with deeper tread depths than standard all-season or summer tires. Some winter tires even have a series of wear bars molded in their tread pattern indicating approximately 6/32″ remaining tread depth.

What is the right size for my vehicle?

A tire’s first requirement is that it must be able to carry the weight of your vehicle. No matter how good a tire you select, if its capabilities are “overworked” just carrying the load, it will have little reserve capacity to help your vehicle respond to quick emergency. So when you are in the selection process, make certain that your new tire’s size is designed to carry the weight of your vehicle! Don’t undersize.

The other size consideration is overall tire diameter. Since many of the functions of today’s vehicles are highly computerized, maintaining accurate speed data going into the computer assures accurate instructions coming out. And an important part of the speed equation is your tire’s overall tire diameter. For cars and vans, staying within a ±2% diameter change is desirable.

Some plus sizing on pick-ups and sport utility vehicles (SUVs) has traditionally utilized sizing outside of this ±2% diameter allowance. The critical factors of concern when changing from the Original Equipment size are: 1. to ensure the tires carry the load (and potential load) for the vehicle; 2. to ensure the vehicle stability has not been compromised; 3. the acceleration and braking have not been compromised by the change; and 4. there is no interference of the tires with the body, suspension, or chassis of the vehicle.

Most tire dimensions can be calculated. See the section on “How do I calculate tire dimensions?”

While at first a ±2% diameter increase or reduction in tire diameter may sound very limiting, in most cases it allows approximately a ±1/2″ diameter change.

Additionally to help with the selection of substitute sizes, a system called “Plus Sizing” was developed. Use Plus Sizing to take into account the diameters of the available tires and the wheels, and this helps select the appropriate tire width that ensures adequate load capacity. Maintaining the tire’s overall diameter helps maintain accurate speed data going into the computer.

How many tires do I need?

Since tires affect the personality and performance of your vehicle, all four tires should be as identical as possible or handling problems may arise. If your tires don’t match, it is possible that one end of your vehicle won’t respond as quickly or completely as the other, making it more difficult to control.

Just one tire?

If your tires have a lot of remaining tread depth, but you need to replace just one that has been damaged by an accident, road hazard or a vandal, you should replace it with a tire that exactly matches the others. Select a replacement tire of the same brand, line, size and speed rating. While there may be a less expensive tire available, it wouldn’t be a bargain this time because it would be different than the other three tires on your vehicle.

A pair of tires?

If two of your tires have a lot of remaining tread depth, but you need to replace the other two because they were damaged or have worn out, you should replace them with a pair of tires that come as close as possible to matching your existing tires. While identical new tires are desirable, others of the same size and type can also provide good results. Only consider selecting new tires that are from the same tire category as your existing tires. New tires should be installed on the rear axle.

While your vehicle is being serviced ask your mechanic why one pair of tires have worn faster than the other pair. Was it caused by a lack of tire rotation, out-of-spec wheel alignment or loose mechanical parts? Once the problem has been found, it can be corrected before it damages your new tires. Keep in mind that your ultimate goal is that all of your tires always wear out at the same time so they can be replaced as a set.

A set of tires?

If all of your tires are wearing out together, you have the greatest flexibility in tire selection. If you were happy with the original tires, simply replace them. If you want longer treadwear, a smoother ride or more handling, there are probably tires that will help you accomplish that.

How do I calculate tire dimensions?

Example…185/60R14 85H or 185/60HR14 The first number is the width of the tire in millimeters, measured from sidewall to sidewall. To convert to inches, divide by 25.4 in the example above, the width is 185mm or 7.28″.

The second number is the aspect ratio. This is a ratio of sidewall height to width. In the example above, the tire is 7.28″ wide, multiply that by the aspect ratio to find the height of one sidewall.

In this case, 185×0.60=111mm or 7.28″x0.60=4.36″.

The last number is the diameter of the wheel in inches.

To figure the outside diameter of a tire, take the sidewall height and multiply by 2, (remember that the diameter is made up of 2 sidewalls, the one above the wheel, and the one below the wheel) and add the diameter of the wheel to get your answer.

185mm x .60=111mm x 2=222mm + 355.6mm (14″) = 577.6mm or 22.74″ (((185 X .60) X 2) / 25.4 ) + 14 = 22.74

A shorter formula exists that makes this a little easier. It is Section Width (SW) times the Aspect Ratio (AR) (full number) divided by 1270 (a constant that takes care of the percentages, the two sidewalls, and the conversion to inches), plus the rim diameter (RD) (in inches), which gives the Outside Diameter (OD). Here is how it would look with the same example above.

((SW X AR) / 1270) + RD = OD

((185 X 60) / 1270) + 14 = 22.74 inches