Get a Grip

Let’s face it—compared to a shiny engine or snazzy-looking instrument panel, tires are hardly the most glamorous part of a car. Yet they are always among the most critical of a car’s systems because they perform a critical task –the car’s actual contact with the road. Keeping your car planted on the road (or churning through some soft, treacherous surface) and pointed where it should be is the ultimate job of the tires.

Caring for Your Tires

So how should you take care of these four round lifesavers?

First of all, make sure they’re properly inflated. Each time before you enter your car, glance at all four tires to make sure they’re not soft or flat. Fixing a flat is much easier in a parking lot or garage than out on the open road. At least once a month, or better yet, every two weeks, check the tire pressures using an accurate pressure gauge. Visual inspection is fine for a safety check with each trip, but tires that have lost quite a bit of air may still look round even though they’re severely underinflated.

To get an accurate reading, check the pressure when the tires are “cold”—that is, if the car has been parked for at least four hours and has been driven for less than 1 km since. Tires that are “warm” or have just been driven will normally have a pressure a few kilopascals or pounds per square inch higher than the standard. This is normal and you shouldn’t bleed air from the tires. If a tire is below its recommended pressure, take out your foot pump and start pumping! If the legs are too tired or lazy, you can head to the gas station to charge up with air. Remember to check your spare tire, too. The spare should be inflated 6-8 psi higher than the road tires, as they will inevitably lose some air when not in use. A lot of gas station pressure gauges are not accurate, so bring your gauge. With a gauge, you will also be able to note if a tire is slowly deflating. If it is, have it checked for punctures as soon as possible.

When a tire loses its air pressure on the road, it’s likely to lessen your control of the vehicle. The steering wheel may pull to one side and become difficult to turn. If deflation is severe, the affected tire may even be dismounted from the metal wheel. Unless your name is Bond, James Bond, this is a dangerous occurrence. If you do feel a loss of handling or steering control, don’t jerk your wheel suddenly or slam on the brakes. Just ease off the gas, apply the brakes gently and bring your car to a halt. If you’re on a major road or highway, better to drive at walking speed (about 2-3 km/h) and get off at the nearest exit or intersection, then find a safe place to change your tire. Better to ruin one tire than risk your life changing the flat with cars whizzing by at high speeds.

Proper tire inflation will help you maintain good fuel economy. You can even use tire inflation to slightly improve your vehicle’s handling characteristics. If you increase the inflation of your front tires 1-2 psi above the recommended level, you can attain a crisper steering response, perhaps at the expense of a bumpier ride. The improvement will be modest at best, but the cost is just a few pumps of your foot. Take note that you should never exceed the maximum allowable pressure and that your suspension may suffer greater wear.

Rotating your tires (changing their position on the car, say from front to rear) will help even out their wear and substantially increase their lifespan. Rotation is usually done every 10,000 km. Rotation schedules and directions vary by model and variant, so check out your owner’s manual.

Identifying Your Tires

All of the information that you normally need to identify your tires are molded into the tire’s sidewall. These include comparative grade designations for treadwear, traction and temperature for all passenger car tires, except deep tread, winter-type snow tires; space-saver or temporary use spares; or tires with normal rim diameters of 12-inches or less.

The Uniform Tire Quality Grading (UTQG) is the U.S. system of grading tires. It doesn’t represent a safety rating but merely a means to compare one tire to another.

TREADWEAR: 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 example, a tire graded 150 would wear one and a half (1½) times as well on the government course as a tire graded 100. The relative performance of tires depends upon the actual conditions of their use, however, and may depart significantly from the norm due to variations in driving habits, service practices and differences in road characteristics and climate.

TRACTION – AA, A, B AND C: The traction grades from highest to lowest are AA, A, B and C and they represent the tire’s ability to stop on wet pavement as measured under controlled conditions on specified government test surfaces of asphalt and concrete. A tire marked C may have poor traction performance.

WARNING: The traction grade assigned to this tire is based on braking (straight ahead) traction tests and does not include cornering (turning) traction.

TEMPERATURE – A, B and C: The temperature grades are A (the highest), B and C, representing 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 material of the tire to degenerate and reduce tire life, and excessive temperature can lead to sudden tire failure. The grade C corresponds to a level of performance which all passenger car tires must meet. Grades A and B represent higher levels of performance on the laboratory test wheel than the minimum required by law.

WARNING:
The temperature grade for this tire 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.

Upgrading Your Tires

All tires, from the skinniest space-savers to the widest high-performance rubber, maintain contact with the road through a small area of rubber called the contact patch. The contact patch is the small rectangular area of the tire that’s actually pressing down on the road, and it’s usually no larger than your handprint. Upgrading to a wider tire also widens the shape of your tires’ contact patches, with potential benefits in cornering ability and feel. Low profile tires have a shorter and wider contact patch that translates to more responsive handling and cornering stability.

The other main reason for upgrading to a wider and larger-diameter tire, and sometimes the one given more importance, is of course the killer looks of a big-wheel, low-profile setup.

When upgrading to a larger-diameter wheel and wider-tire combination, it’s best to maintain the total outside diameter of the original wheel-tire combo. Keeping the outside diameter is ideal because the car’s speedometer, odometer, ABS sensor calibration and ride height were calibrated or determined using the original OD.

To compute for the outside diameter of the original wheel-tire combo, take the tire width and multiply by the aspect ratio to get the sidewall height. Multiply this by two (since there’s a sidewall on either side of the wheel), and add this to the diameter of the wheel. Remember to convert the wheel diameter to mm. (1 inch=25.4 mm). For example, a 195/65 R 15 tire will compute as:

195 mm (0.65) = 126.75 mm
126.75 mm x 2 = 253.5 mm
Plus 15 inches * 25.4 mm/inch = 381 mm
Outside diameter = 634.5 mm

A Plus One application means a diameter increase of one inch, with correspondingly lower-profile rubber. If Upgrading to a 16-inch wheel, for example, to a 205/55 R 16 :

205 mm (0.55) = 112.75 mm
112.75 mm x 2 = 225.5 mm
Plus 16 inches * 25.4 mm/inch = 406.4 mm
Outside diameter = 631.9 mm

The difference in the outside diameter is 0.4%, which is an acceptable difference.

If you compare that to an upgrade to a 235/60 R 16, where the outside diameter will be 688.4 mm, or a difference of 8%. That increase in diameter will wreak havoc with your car’s suspension and other settings.

Plus Two means a two-inch increase in the wheel diameter with a correspondingly lower profile or aspect ratio.

Keep in mind that a lower-profile tire with a larger wheel will be more susceptible to damage from potholes and curbs. That’s because an inflated tire is able to absorb shocks and impacts better than a metal wheel. Damage is even more likely if the wheel is light-alloy. Given the poor state of a lot of our city roads, be prepared to suffer wheel damage if you opt for very low-profile tires.

A large-diameter wheel and tire combo will also tend to be heavier than its small-wheel counterpart. A metal wheel certainly weighs more than rubber and air. This increase in unsprung weight, meaning weight that’s not supported by the car’s springs, may be detrimental to the car’s suspension response.

Changing to a lower-profile tire without increasing the size of your wheel is not a good way to go. Your car will ride lower and the under-filled wheel will look a bit silly. One good way to increase performance and/or ride comfort is to maintain your car’s original tire size and select a more performance-oriented brand or comfort-oriented variant. Finally, let’s take a look at the tires’ speed rating and load index.

Speed Rating and Load Index

When upgrading always choose a tire with equal or higher speed rating and load index. For example, if a tire is rated 89H, pick a load index of 89 or higher, and a speed rating of H or higher.

Speed Rating

Speed ratings indicate the maximum speed at which a tire can be run, and they apply only to tires that have not been damaged, overloaded, altered, or underinflated). A tire that has been repaired after a puncture is no longer guaranteed to retain the tire manufacturer’s original speed rating.

Previously, a V-speed rated tire with the “V” branded “within” the tire size (as in P205/65VR15) indicated that the tire was capable of 210+ km/h. Today’s new V-speed rated tires are always identified in the tires service description (as in P205/60R15 90V) and this indicates maximum speed of 240 km/h.

SUV and light truck tires have “function ratings” after the brand name. A/T is All-Terrain, M/T is max traction mud tire, H/T is Highway all-season tire and S/S is Street Sport tire.

By Jason Ang | Photos By Jason Ang and Ulysses Ang
Addition Material and Photos Courtesy of NHTSA
Originally Published in the October 2002 Issue