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I inflated my tires based on a good tire pressure gauge (well I think its good?).
After a mile or two the TPMS gave a pressure reading and I was 3 psi too high.
So my question is how accurate are these TPMS, or are they just meant to be a very rough check so the tires are not a long way over or under inflated.
My other cars do not give a reading they just tell you if it is very low. Thanks Dave
 

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How hot was it out? Heat will raise the pressure, even after a few miles. Also, when the tires start to turn that creates heat too. You ALWAYS check the tire pressure when the car is cold and hasn't been driven. Here is an example. If one of my front tires are in the sun the pressure will read higher than the tire that is NOT in the sun by almost 2 pounds. I would re-check the tire pressure in the morning before you go to do any errands. Higher tire pressure after you have been driving is totally normal.

Best regards,

-Mike
 

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Modern TPMS sensors are accurate and they can distinguish a change of 1 psi. Ghost is right about all the things that vary tire pressure. With all measurement instruments however, they are only as good as their calibration. Could anyone on this forum tell you whether your gauge or TPMS is correct? Maybe. I would try using the Gay-Lussac's Law which relates temperature and pressure of a gas at constant volume. It's not perfect but it's a start. Let's assume the tires volume is constant; they're pretty stiff once inflated, so a fair assumption. The Law then states that:
Pressure_1 / Temperature_1 = Pressure_2 / Tempurature_2
Say you filled your tires up to 36 psi in the morning when it was 60 F outside, what is the pressure in the tire later that day when it reaches 90 F.
So let's calculate Pressure_2
36 psi * 305.37 K / 288.71 K = ~38.12 psi
So an increase in 30 F outside temperature has given us 2 psi extra pressure in our tires. This is without driving on them. Add another 15 F above ambient temperature to the tires after driving and you have your 3 psi extra.
So, my guess is that both TPMS and gauge are probably accurate as long as my model here somewhat describes your situation.
 
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