I just got them last week and I set them to 35psi, then I lowered them to 32 and it gave a better ride and I think they will wear better since my old ones were at 35 and they wore on the insides.

Anyway, I go to check them today after driving (it's about 60* out) and they were all around 35-38! where did this extra air come from? I used the same gauge both times and it's a quality one.

is it because they were hot? I let them all down to 32/32 psi but when is the right time to check them? after the car has sat in the cool garage? or should checking them after driving be accurate?

 

The tire pressure does go up when the tires are hot. As far as the right time...most folks do it while cold...say first in the morning. Just as long as you do it consistently. It's easier to judge then when the tires are cold than known when they are heated up enough to get the "hot" reading consistently.

Personally, I check mine in the morning, and I run 34 psi cold. Wear seems to be pretty even...but that's a personal preference.

 

Check the tire pressure label on your door. Those are the recommended "cold" pressures. The label then says "add 4psi when hot".

Pressure is directly proportional to air (absolute) temperature (degrees Rankine). Add 50deg to the air temp and the pressure in the tire will increase about 10%.

 

rule of thumb. For every 10 degrees the temp goes up your tires will gain 1lb. and vice versa.

 

Holy cow, you make my job easy...

 

Here's the calculation for a tire that measures 32psi at 60degF (520degR), and has the temperature elevated by 50degrees to 110degF (570degR):

((110 + 460) / (60 + 460)) X 32 = (570 / 520) X 32 = 1.09615 X 32.0 = 35.1psi.

An increase of 50degF caused the pressure to increase 3 psi.... about 10%