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Tire pressure question


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I have an '08 Boxster S with 19" wheels.  I check my tire pressure with a gage every copule of weeks.  I have noticed that the tire pressure on the gage is always higher than what the TPMS reads.  I was concerned about this so I looked it up in the manual.  The manual points out that the recommended pressures are for cold tires at 68F and that tire pressure changes with temperature.  It then goes on to say that the TPMS takes into account the relationship between temperature and pressure.

 

So does the TPMS tell you what the pressure in the tires would be if the temperature were 68F?  When it's 98F outside I don't care what the pressure would be if it were 68F, I care what the pressure is now.  I fill my rear tires to the recommended 36psi based on todays conditions.  Am I looking at this wrong?

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The function of TPMS is to determine if your tire pressures have dropped.    As you may remember from high school physics,  the Combined Gas Law states that pressure increases with temperature.  Since you don't want TPMS triggering a low pressure warning just because it happens to be -10 degrees outside, the system has to compensate for the temperature.  It is this temperature compensated pressure that is displayed on the dash.

 

I use the dash pressure as a reference.  If it shows I'm off by 2psi, I use my gauge to see what pressure that reads and set the pressure to 2psi over that.   Works a treat :thumbup:

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