“I always wondered what was meant by Max pressure”
Let’s try to relate this to blood pressure.
If your doctor told you that he/she considered…let’s say…150/85 to be the maximum blood pressure before he/she prescribed medication to lower your blood pressure, would that mean that 150/85 is a desirable blood pressure?
(Hopefully you know that the correct answer is, “no”)
So–the maximum pressure that is listed on the sidewall of your tire is just that–a maximum–and not a desirable pressure.
The correct tire pressure for your vehicle is most likely listed on the placard affixed to the driver’s door jamb.
If you don’t find it there, your Owner’s Manual will have the necessary information.
Check your tire pressure at least once a month, after the car has been sitting for at least a few hours. If you check the tire pressure too soon after driving, it will be temporarily elevated.
“I’ve also head that tire pressure increases 1 PSI for every 10 degree increase in temperature. Is this true?”
It’s a rule of thumb that’s approximately true as long as you are talking about normal car tire temps and pressures. It doesn’t necessairly hold true for ten speed bike tires inflated to 120 psi or dragster tires inflated to 4 psi. It also assumes a normal range of weather temperatures.
A more precise way is to change the Farenheit scale to an absolute temperature scale, such as the Rankine temperature scale, an absolute scale that uses Farenheit scale divisions. Since absolute zero is minus 458 F, zero F is 458 degrees Rankine.
So you inflated your 10 speed bike tire to 120 psig when it’s zero, (458 R) outside and it warms up to 40 degrees F (498 R). You take the ratio of those two temperatures 498/458 and multiply by the absolute pressure, not the gauge pressure in that bicycle tire, the absolute pressure is 120 plus 14.7 psi or 134.7 psi absolute.
Your new pressure is 498/458 X 134.7 = 146.5 absolute. Now we subtract 14.7 psi to go back to gauge pressure and the new pressure is 131.8 psig, a gain of 11.8 psi.
See how that disagrees with the rule of thumb? That’s because we were using pressures that are out of the range of normal car tire pressures.
A better rule of thumb would be each 10 degrees raises the pressure by about 2 percent.
Good point. The rule of thumb is definitely designed for passenger car tire pressures - and it doesn’t work for higher pressures regradless of the application. However, it is difficult enough to get folks to check their tire pressures, so I hope you’ll forgive all of us if we state it in very simple terms so it will be easy for the average joe to relate to - after all, that is the point of all this.
Oh snap! Absolute zero is minus 459.67 F, not minus 458 F, so my answer was completely wrong!
Another point. If you live in Taos, NM which is about 7000 ft above sea level, and you decide to visit Corpus Christi, TX, which is on the Gulf of Mexico, you might want to check your tire pressure when you stop for gas along the way. The increase in atmospheric pressure will show up as a reduction in gauge pressure in the tire.