Great Question

Question:
Hello,

I'm writing to inquire about proper measurement of tire pressure for my car. I'm mostly concerned about when the tire pressure should be measured. My vehicle placard calls for 35 psi, (or 240 kPa) which is about what was in the tires when I purchased the car. I have inflated the tires to a 35 psi when the tires are cold (ie: approximately 15-20 degrees celcius, after not being driven on). My question is whether this is the correct baseline to use with respect to the vehicle placard suggestion of 35 psi. For instance, on a nice sunny day driving 100 km/h on a highway for a couple of hours the tire pressure will be up to around 38 psi while driving. Is this something I should be concerned about? In a nutshell, does the 35 psi on the placard refer to the ideal pressure when cold, or the ideal pressure when driving?

Thanks

Answer:

Your question is a good one. It is important that when you measure and adjust your tire pressure, that you do it when your tires are cold which means the vehicle was parked overnight or at least 3 hours or it has not been driven for more than two kilometers. The recommended pressure on the vehicle placard assumes a cold tire pressure measurement. The fluctuation in pressure that you observe after driving on the highway is nothing to worry about and it normal as the flexing of the tire creates heat that will expand the moisture in the air that is in the tire. The 35 psi you see on the placard is the ideal pressure for the vehicle when it has been parked or not driven for more then 2 km.