Join Date: May 2013
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Several variables to consider. Was the 40 psi pressure measured right after driving on a highway, or after the car was parked for a while? Was it a particularly warm day, average day or really cold weather?
The pressure on the sticker is cold tire pressure, meaning that this is the pressure you should have at whatever is the ambient temperature after a long rest period - typically first thing in the morning, before driving the car.
So, if you had 40 psi after several miles on a highway on a warm or average day, then I would say it's normal. The pressure will probably be around 32-34 psi if you measure it on a next cold morning. You get ~1 psi for each 10 F ambient temperature, and the difference between cold (i.e. after long rest) and warm (i.e. within 1 hr after highway drive) tires is typically another 3-5 psi.
That's why you really should check tire pressure first thing in the morning, before driving. Even a couple of miles on a road will make a difference.
As for tire damage, you don't need to worry. Most tires are rated 44 psi cold, meaning that that they can withstand around 55 psi or more when warm/hot. Some tires tend to wear faster in the center when over-inflated, but it really depends on the tire construction.
Most people actually drive with under-inflated tires - just have a look at several cars on a parking lot and you will notice that most tires are more worn toward the edges then in the center - a clear indication that the tires are under-inflated. This is because most people check tire pressure at a gas station, not realizing that the tires are warm, while the manufacturer's recommended pressure is applicable to cold tires only. Actually most owner manuals state that you should NOT lower the pressure if you find it higher than recommended if you check the pressure immediately (less then one hour) after driving. But who reads manuals, right?