Tires naturally lose about 1 psi (0.068 bar) per month under normal conditions, but other factors can cause more dramatic swings in air pressure.

In cold weather, for instance, motorists need to fill their tires more often because a 10º F (4.5º C) temperature drop generally causes a loss of 1 psi. So an extreme cold snap can leave a tire noticeably underinflated, decreasing fuel economy, shortening the lifespan of the tread and increasing the likelihood of a tire failure.

It takes about 20 minutes of driving before tires begin to warm up, which will boost air pressure, but only slightly. Still, tire industry experts say it’s better to add more air in winter time, and it’s safer to keep tires overinflated than underinflated.

By law, every vehicle in the U.S. has a “placard level,” which the auto maker recommends as the optimal inflation threshold for tires. That information appears on the driver’s side door jamb, in the glove box or underneath the lid of the center console.

Tim Hannon, TRW Automotive’s global technical sales lead-radio frequency products, says other factors can affect tire pressure, as well, including sunlight and changes in altitude and barometric pressure. A drive from Denver to sea level, for instance, would result in the loss of several pounds of air pressure.

Hannon also warns that a vehicle heavily loaded with cargo and occupants can require up to 8 psi (0.5 bar) of additional air to meet the vehicle’s recommended placard level.

Joseph Kanianthra, a safety consultant and former associate administrator at the National Highway Traffic Safety Admin., says underinflated tires are prone to skidding and loss of control, especially on curves and when changing lanes. They also are more likely to hydroplane on wet surfaces and require increased stopping distances.

If a vehicle is equipped with tire-pressure monitoring, Kanianthra says it is important for the owner to replace the batteries that power the systems. The batteries can last between six and 10 years, experts say.

Kanianthra says battery-less sensors are being developed by some suppliers.

tmurphy@wardsauto.com