Posted by **Mary** on Monday, October 5, 2009 at 12:36am.

Why is the air (atmospheric) pressure of 14.7 lbs/in2 not included in the pressure gauge reading for a tire?

- Physics - Tire Pressure -
**Mary**, Monday, October 5, 2009 at 12:41am
I made an error - Why is the atmospheric pressure of 14.7 lb/in2)not added to the pressure of the tire gauge when computing the weight of the vehicle?

- Physics - Tire Pressure -
**drwls**, Monday, October 5, 2009 at 1:38am
You can only get an approximate weight of the vehicle by multiplying the tire gauge pressure by the area of the tires in contact with the road.

It is appropriate to use the gauge, rather than the absolute, pressure in such calculations because atmospheric pressure acts on the entire exterior of the car except the road contact area. This has the effect of creating a buoyant force that equals the reduction in weight that you get by using the gauge pressure in the weight calculation.

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