Tire pressures during temperature swings
I see a 3-4 PSI rise in my Ram 2500 tires from a cold start to highway driving after 30 minutes. During the day, if starting off from a cold location, to the hottest destination, do you account for...
Forum Discussion
Harvey51 wrote:
The physics of pressure and temperature is PV = RT
Actually the Ideal Gas Law is PV = nRT where "n" is the number of moles (a measure of the number of molecules of gas). :R
"R" is a universal constant and the value of R depends on the particular gas. "T" is the temperature in Kelvin (called the "absolute" temp.). 70F for ex., = 294K.
It gets a bit more complicated because air is a mixture of gases - oxygen, nitrogen, argon, helium, CO2 and a few others. As well, air has water vapor in it and pressure can also be affected by the dew point. The volume of a tire will also be affected somewhat by the atmospheric air pressure (think weather balloon).
Gay-Lussac's law is more applicable for tires because n & R don't significantly affect the above gas law equation and P1/T1 = P2/T2 can be used. If you started your day at 70F and the tire air temp went to say 140F (333K), the increase in pressure would be 333/294 = 1.133 times (13 percent increase). Simply not much to worry about. Note that tire air temp. will be somewhat higher than ambient. And don't forget that the max. pressure on the sidewall of a tire is cold pressure.
Having said all that, if you really need to know the affect of temp., all you need to know is the general rule of thumb approximation, a 2% increase in pressure for every 10F increase in temp.
