Air Pressure vs. Dry Performance

Most drivers realize that tire load capacity is determined by tire size and inflation pressure. Larger tires and higher inflation pressures provide more load capacity, while smaller tires and lower tire pressures provide less.

Correctly inflated tires receive appropriate support from the contained air pressure to provide an even distribution of load across the footprint and help stabilize the tire's structure. And while most drivers recognize that this has a significant impact on tire wear, rolling resistance and durability, only a few realize underinflation also has a noticeable influence on how quickly and precisely the tires respond to the driver's input.

In order to evaluate the influence of inflation pressure on response and handling, the Tire Rack conducted a Performance Test Track Drive, comparing properly inflated tires to purposely underinflated tires. We used 2003 BMW 330Ci coupes, and installed P225/50R16 tires on 16x7.5" wheels. We tested new, full tread depth tires.

We chose Goodyear Eagle GT-HR High Performance All-Season radials that were developed to blend good treadwear, responsive handling and dependable traction. One of the Eagle GT-HR's highlights is its internal construction that features Goodyear's RaceWrap Construction Technology developed for the Eagle Race tires used in NASCAR competition. RaceWrap Construction Technology brings a casing ply down the sidewall at a slight angle, wraps around the bead and returns it all of the way up the sidewall until it ends under the edge of the steel belts. This slightly angled, two-ply sidewall enhances steering response and handling stability.

The tires installed on one of our BMW 330Ci test cars were inflated to the vehicle manufacturer's recommended inflation pressures of 29 psi front and 33 psi rear, while the other car had its tires inflated 30% lower (20 psi in the front and 23 psi in the rear). We chose 30% underinflation because it was the percentage of loss initially established by the US DOT at which passive pressure monitoring systems must warn the driver of low inflation pressure on future cars.

The first part of the test was visual. We asked the drivers to look at the tires and decide which of the two vehicles was equipped with the underinflated tires. While perhaps this visual test might have been easier with taller tires of the past, today's low profile tires fitted to the BMWs demonstrated how difficult it has become. The drivers agreed that the tire appearance alone did not provide irrefutable confirmation of the tire pressure contained inside. You can't use your eyes as a tire pressure gauge.

Can you easily identify which tire is 30% underinflated with your eyes? Here is what they would look like in the morning as you walked to your car in the garage.


[You must be registered and logged in to see this image.]

The next test was run on our test course to confirm the influence of tire pressure on the tire's performance at its limit. While the drivers thought that the properly inflated tires provided responsiveness and predictable handling, they quickly realized that the same tires in an underinflated state left a lot to be desired. The underinflated tires required more steering input to initiate maneuvers and were slower to respond. The underinflated tires also felt out of sync during transitions; instead of moving in unison, the rear tires' reactions lagged behind the front tires, resulting in a detached sensation being transmitted to the drivers.

The underinflated tires delivered acceptable steady-state cornering force once they stabilized on our test track's skid pad, but the car was uncooperative anytime it was asked to change directions. It proved to be over 2 seconds slower around our test course (2 seconds represents about a 7% loss of handling performance).

In other words, the performance that tire manufacturers build in, low tire pressures can take away.

Adjust your tire pressures as indicated on the vehicle tire placard or in the owner's manual. Check your inflation pressures at least once a month and before highway trips.