The crew over at Consumer Reports' Cars blog weigh in on the subject of tires today. Tires are probably one of the single most important variables of a vehicle's performance. They impact acceleration, braking, handling, ride, and noise. Tires also have an impact on fuel economy. If you've ever had a balloon you know that over time, the air will seep out of the balloon and it will slowly deflate. Tires work the same way. Over time the air molecules will seep between the rubber molecules of the tire. Left unattended, a properly inflated tire will end up under-inflated. Under-inflated tires cause a variety of problems including un-even tread wear, loss of lateral and longitudinal grip leading to poor steering, handling and longer braking distances.
Low tire pressure also causes increased noise and rolling resistance. Increased rolling resistance has a negative impact on fuel economy, so it's important as gas prices rise to make sure that you check your tire pressures with a gage at least once a month. Unlike the old days of bias ply tires, it's hard to tell from visual inspection if a radial tire is low on pressure until it's almost completely flat. Without a gage it's almost impossible to distinguish a radial at 15 psi or 35 psi by sight.
One thing to avoid is anyone trying to sell you nitrogen for your tires. Nitrogen is used in race car tires because it has less thermal expansion than regular air. Air however, is already almost 80% nitrogen. Race cars are very precisely aligned, and suspension components are regularly replaced as they wear. Race engineers are constantly monitoring tire temperatures and pressures. Small changes in alignment and balance caused by expanding tires has a noticeable impact on high end race cars. On the road where cars goes tens of thousands of miles between suspension component changes, the difference attributable to filling tires with nitrogen are negligible. The gross changes caused by differences 10 psi or more. are measurable so save your money and the best advice is to just keep your tires at the recommended pressure.
[Source: Consumer Reports]