Like good little people, we keep an eye on our tire pressure via the Tire Pressure Indicator in the instrument cluster.

If low, the wife monitors the readout whilst I tag a bit of air into the tire to get the correct reading (34psi - cold).

Well, we just had an oil change, and it came back with 39psi in both front, and 33/37psi rear (it went in at 34spi all round).

So, I broke out the good old fashioned manual tire pressure gauge (two of them to double check and be sure), and started deflating to 34psi... a short time on the right rear and it was at about 34¼psi on the manual gauges (both read the same) and the on-board readout said 36psi... another quick blast and I had 34psi on the gauge, and 33psi on-board... odd, but OK, on to the front.
The front left (only starting 2psi higher than the rear right) seemed to take a lot more time to get down to where it ought be, and then also jumped 36psi to 33psi in a short time.
The front right took even longer, to drop, and the on-board only went from 39psi to 37psi, even when at 32½spi on the manual gauge.

I tried cycling the ignition, start/stop the engine, rock the car, anything to try and get it to budge off 37psi, without any joy.
I'm letting it go for now, and will check it again tomorrow evening, after a drive.

Now for the real question; How accurate is the on-board tire pressure system supposed to be?
And how quickly should it respond to actual tire pressure change?

 

You need to be going at a steady speed when checking the tire pressure from the car computer and I believe itsabove 45mph maybe faster and it gives you a more accurate psi

 

yeah the system does not work by providing constant real time up date of PSI. There is a sensor reader located in the wheel well some where (location varies by car). it only reads the sensor and updates the Body Control Module when the sensor (located in the valve stem) passes by the reader. so if the reader is located 12 o'clock and the valve stem is located at 6, it isn't close enough to read it.

But if you have a speedway gas station around use the air pumps there. they are digital now so you set your desired PSI, hold on the hose, and its all automatic from there. I set my dart with 37 on fronts
34 on rears. and the read out on the dash matches the read out on the air pumps.

(FYI i'm an electrical engineer that has worked at various suppliers since 2002, and have tested and worked on these systems)

 

OK, that answers a lot for the static reading.

The thing I've now found is the inconsistency/variation of readings.
After happily setting TP with a manual gauge to 34PSI all round, and taking a drive to get the system to update, it read 31/33 & 33/33 cold.
But warm it reads 37/36 & 36/36.
I can see a 1PSI discrepancy between the system and a manual gauge... no big deal.
3PSI (or a little more) cold to hot seems normal.
But a 6PSI swing on the front left seems odd.