TPMS accuracy
11-11-2022, 08:09 AM
#17
So the actual tire pressure will vary about 1.5 PSI for every 10F. As you drive the tires heat up so the pressure will increase.
If you pull up the pressures in the display by pushing the lever on the left up a few times (on the 987.2, at least - I assume the 997.2 is the same?) that tells you the actual pressures currently in the tires. If you pull up the pressures by going into the TPMS submenu (push forward, select TPM, select Info Pressure...this only works when fully stopped) it'll tell you the difference between your current pressure and the programmed spec. This second menu is corrected from the temperature inside the tire to the spec at 68F, which isn't really made clear.
Example: the spec for a tire is 32 PSI. You fill it to 33 PSI in the garage at 68F. The first menu will read "33" and the second menu will read "+1", because it's 1 PSI over spec.
Now you park the car out in the cold overnight. In the morning it's 48F. If you pull up the pressures, the first menu will now read "30" (3 PSI lower, because the pressure dropped due to the 20F lower temperature) but the second menu should still read "+1" because it is temperature corrected. In theory this lets you do things like fill the tires when they're hot (using the values in the second menu, if it says "-1" then just add 1 PSI to whatever it is). In practice I've found it to be too inaccurate to be useful for anything but emergencies.
If you pull up the pressures in the display by pushing the lever on the left up a few times (on the 987.2, at least - I assume the 997.2 is the same?) that tells you the actual pressures currently in the tires. If you pull up the pressures by going into the TPMS submenu (push forward, select TPM, select Info Pressure...this only works when fully stopped) it'll tell you the difference between your current pressure and the programmed spec. This second menu is corrected from the temperature inside the tire to the spec at 68F, which isn't really made clear.
Example: the spec for a tire is 32 PSI. You fill it to 33 PSI in the garage at 68F. The first menu will read "33" and the second menu will read "+1", because it's 1 PSI over spec.
Now you park the car out in the cold overnight. In the morning it's 48F. If you pull up the pressures, the first menu will now read "30" (3 PSI lower, because the pressure dropped due to the 20F lower temperature) but the second menu should still read "+1" because it is temperature corrected. In theory this lets you do things like fill the tires when they're hot (using the values in the second menu, if it says "-1" then just add 1 PSI to whatever it is). In practice I've found it to be too inaccurate to be useful for anything but emergencies.