Q: Tire Pressure

asked by on February 28, 2018

My tire recommended pressure is 35, but everytime I check it in the morning it says 39. Should I relase some of the air or its okay?

My car has 1400 miles.
My car has an automatic transmission.

Tire pressure is measured and set when the tire is stone cold. However, cold tire pressure tire, once set when the tire is at a specific ambient 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, further 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, however, it is 70 degrees and the tires are in direct sunlight. Inasmuch as tire pressure will increase 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. In your circumstance, if you are well within a "season" (say spring) and the daily temperature is around 50-70 degrees, if you set the tire pressure to 35 PSI at 65 degrees, you will OK on average. However, if it’s 70-90 degrees every day and you come out in the morning when it’s 70 degrees out and your pressure is already 39 PSI you should reduce your pressure to 35 PSI because as the day wears on, and temperatures increase, your tire pressure will also increase and then be too high. Please let us know if you have further concerns or questions as we are always here to help you.

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