Q: tire pressure

asked by on February 19, 2017

Yesterday the psi in my car tires read 35 psi in all 4 tires..i checked them again the next day and they all read 44 psi or higher! I have owned over 40 cars in my life and never ever once had i had to let air out!..what gives?

My car has 27454 miles.
My car has an automatic transmission.

Car manufacturers require that tire pressure be set to the required psi before the vehicle is driven on the roadway. Tire pressure has to be measured and set before the car has operated on the roadway because friction from operation raises the internal temperature of the tire and hence the air pressure within. This manufacturer specification is referred to as cold tire pressure and is the required pressure documented on the vehicle information placard on the door pillar. However, cold tire pressure tire, once set at a specific temperature, will go on to vary up and down as ambient temperature goes up and down. Furthermore, if the sun is shining on a black tire, the internal temperature within the tire will increase significantly above what you measure as ambient, increasing the pressure. To illustrate, let’s say that yesterday it was 25 degrees out and the car was parked in the shade. The cold tire pressure was set at, or observed as, 35 psi. Today it is 70 degrees and the tires are in direct sunlight. Inasmuch as tire pressure increases 1 PSI for every 10 degree increase in ambient temperature, you will have a 4.5 psi increase today just due to the change in weather. You report a pressure of 44 and it will be easy to reach that in our illustration if you assume that the tires sat in direct sunlight (increasing the temperature well above the ambient measure of 70 degrees, WITHIN the tire) and/or the vehicle was actually driven on roadways, further increasing the pressure. Please let us know if you have further concerns or questions as we are always here to help you.

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