The best time is when the tires are cold — after the car hasn’t been driven for at least one hour — and when your car is in a garage or shaded area.
That will give you a better reading than if you check the pressure shortly after driving. Air expands as it gets warmer, so the pressure will be higher right after you stop. If you drove for an extended period at highway speeds, it will be several pounds higher.
Bear in mind that the ambient temperature also affects tire pressure. The Rubber Manufacturers Association says air pressure inside tires drops one to two pounds for every 10-degree decline in the outside temperature and rises a corresponding amount when outside temperatures increase. If your car is baking in the midday sun, the tires bathed in sunshine will have higher readings than the ones in shade, so try to check pressure when that isn’t a factor.
Don’t be obsessed with keeping the tire pressure spot on with the manufacturer’s recommended level. Pump an extra pound or two of air into your tires, and you’ll have a little cushion for when air starts escaping, as it eventually does in all tires. A little more air is better than too little. Just don’t overdo it.
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