Tire Pressures Drop as Fall Marches On

October 6, 2008
Fall is upon us, and for most of the country that means dropping temperatures. The air--or even nitrogen for some of you Sam's Club geeks--in your tires shrinks as it gets colder outside.

As morning temps dip lower and lower, checking your tires weekly before setting off, which of course you always do, might result in their pressure dropping week by week. Don't worry--if the change is small, you don't have a leak. Rather, as the air in your tires gets colder along with the weather as fall progresses, its volume decreases. This results in it taking less space in your tire, ergo a drop in tire pressure on the gauge. For every 10-degree drop in temperature, tire pressures drop roughly 1-2 psi (depending upon tire size and overall volume of air inside). Because of this same phenomenon, always check tire pressure in the morning before the tire has heated up from driving duty.

Did you know there's an organization called the Rubber Manufacturers Association (RMA)? Well, there is, and they urge drivers to check their pressures at least once a month for reasons that include both safety and economy. We'd recommend doing it weekly. Even if your pressures are spot-on week after week, it's a good way to commune with your machine, and it builds character to get the hands dirty from time to time. RMA claims that 85 percent of American drivers "do not know how to check their tire pressures correctly." We hope you're not that clueless, but if so it's your little secret, and you can visit betiresmart.org to get with the program.

If your tires are low on pressure, you'll end up sucking down extra gasoline just to keep your ride moving down the highway. Not to mention, you'll be creating intense friction, which results in heat that can ultimately result in nasty things like tread separation. As we all remember too well from the Ford/Firestone fiasco, tread separation is no trifling matter.

Quick note for you more-is-better types: Overinflation ain't so hot, either. While you may eke out a little more mpg riding on rock-hard rubber pumped up to max specs, you'll wear bald spots in the center of the tread as my grandfather, the King Of Overdoing It, found out with his '80s Chevy Suburban diesel (this from the same man who brushed his teeth so aggressively with a "firm" toothbrush that he landed himself a mouthful of root canals).

So get out your quarters and try to find a gas station with an operable air pump that hasn't been run over by late-night drunkards or fallen hopelessly out of service by stressed station owners. Or do what my ultra-organized father does: buy an air pump that plugs into the cigarette lighter auxiliary power outlet in your vehicle. He swears by the mini-compressor that I hear rattling away in his garage from time to time.--Colin Mathews

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