Time to set your tire pressure as fall temperatures drop

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Many people falsely assume that when their tire pressure drops air has leaked out of their tires. Actually, that is not the main reason why tires have lower pressure in the fall.  A long time ago a scientist named Boyle proved that as the temperature in a vessel filled with a gas drops, the pressure does as well.  The cool thing is, it really does not matter what the gas is, air or nitrogen or anything else.

How Much Pressure Does Lower Temperature Steal?

In practical measurements, you will find that as the temperature drops about 10 degree F, your tire pressure will go down about 1 PSI. If you want to get all technical, you can use the formula PV=nRT.  Nobody does that.  Let’s assume you set you tire pressure at 30 PSI when it was 80 degrees back in August.  Now you wake up on a chilly morning, and the temperature is 30 F.  What would your tire pressure gauge read?  It would read about 25 PSI.

How Do I Know What Temperature To Set the Pressure At?

Here in New England, we have temperature swings of 30 degrees on many days of the year. So should you set your tire pressure during the higher or lower temperature part of the day?  All automakers will answer that question exactly the same way.  You set your pressure “cold.”  That means when your tires are cold, not having been driven, and when the ambient temperature where you live is coldest.  So, set it in the morning before you have driven the car.  The proper tire pressure for your car is not on the tire itself, but rather on this label you will find on your driver’s door jam (sideways).  Many modern cars have different tire pressure settings for the front and rear tires.  So read it carefully.

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How Do I Measure The Pressure?

Generally, we car enthusiasts use a gauge. A simple one from any auto parts store will likely work, but spend $20 and get a good one with a needle dial.  Many cars now have the ability to display the actual tire pressure on the trip computer screen.  That is an excellent way to verify the pressure is correct.  Some Nissan and Infiniti models also will allow you to set the pressure using the hose at the gas station, and the car will toot the horn when you are at the proper level.  See your owner’s manual for details.

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Tips For Inflating.  About Nitrogen

My only suggestion is to blast a little air out of the hose before you fill up. You will see that a bit of condensed water will spray out.  Then remember to replace the little cap on the valve.  That valve is called a “Schrader valve” by the way.  Use nitrogen if you prefer, but air is already about 80% nitrogen and the claims that the companies selling nitrogen for passenger car tires make are not based on any scientific principles I learned in engineering school.  Most of them are completely ridiculous.

Check your tires using the trip computer in your car or a gauge monthly. If you find one of the tires is a bit lower than the rest make a note of it.  If it continues after a refill, have the tire checked out at your favorite tire store, garage, or by the dealer.

John Goreham

John Goreham

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