For 1990-2009 cars

# SERVICE GUILD: TIRE PRESSURE/ ASPECT RATIO DESCRIPTION

TIRE PRESSURE

#### Models Affected: ALL ---------------- The following information, although not new, does help clarify some common tire related questions.

#### TIRE PRESSURE

The amount of air pressure in your tires has a definite effect on how the tires will wear, how the car handles and how much fuel it will use.

Underinflated tires may give a smooth ride, but handling and tread life will suffer. Too little air causes the sidewalls and tread to buckle, reducing the area, of the road contact patch and causing a loss of traction, especially on curves. Refer to Figure 1.

The flexing of an underinflated tire increases fuel consumption and leads to excessive heat build-up. That can cause rapid wear and early tire failure. Proper inflation results in maximum road contact, traction and fuel economy.

Overinflation reduces the contact area and hurts traction. Fuel economy may increase, but the ride will be harsh.

#### TIRE ASPECT RATIO

The ratio between the height of a tire, from wheel rim to tread, and its width, from sidewall to sidewall, is known as the aspect ratio. At first, tires were circular tubes with 100 percent aspect ratios. It was discovered that tires handled better when mounted on a wider rim. Tires were then made wider without increasing their height, which tended to put more tread on the road and provided better handling, better load-carrying capacity and longer life. Today's passenger-car tires have aspect ratios ranging from 80 to 50 percent. (See Figure 2.)

Profiles as low as 35 percent are used on special low-pressure racing tires.

General Motors bulletins are intended for use by professional technicians, not a "do-it-yourselfer". They are written to inform those technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions and know-how to do a job properly and safely. If a condition is described, do not assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See a General Motors dealer servicing your brand of General Motors vehicle for information on whether your vehicle may benefit from the information.