The placard value is meant to be at ambient. Manufacturers recognize that as the tires warm up the pressure will increase by a few PSI, and that's accounted for in the information posted on the placard. So best time to check is when the tires are at ambient.

Thanks! Yeah, it happens to me a lot too. I hear a term I haven't heard since college and I instantly feel inclined to go look it up to remember what it is

You're an engineer too. You know how it is, ask us a technical question and we're ready with a dissertation

 

Yeah, that's perfectly normal. All factors being equal, if your car sits outside and the sun is only hitting one side, your tire pressures on that side will be ~1psi higher than on the shaded side.

Having a difference of 2-3psi between tires is no big deal. If the difference is over ~20% your may start feeling handling asymmetry due change of the sidewall stiffness.

 

Use a good gauge. Once you start rolling the TPMS should start reading quickly.

Does it output to OBDII? I ask cause it would be nice to get live output thru the OBDII interface

 

Yes, you can read the information over OBD. The CAN bus(es) are readable via OBDII, but you would need a specially configured device which knows what CAN message and frame to look for. You could also do this with a CANcase, but those aren't cheap.

It is certainly possible to configure an OBD reader to also read the TMPS info.

 

Well like yesterday for example. It read 30. So at the gas station I put some in, while it was in run power w/tires in display. It didn't change. I had no idea how much, so I put more in. Still nothing. So I put more in, till the timer ran out. Then I back out and it reads 40!

 

GM employed a direct system in the GM, Toyota90's (the first that I know of) on the C4 Corvette which IIRC used a piezoelectric generator to power the sensor instead of a battery. The sensors were strapped to the wheels and pretty bulky, about two to three times the thickness of modern sensors.

GM, Toyota and Ford are the only manufacturers I remember seeing an indirect system on (the Windstar or was it the Freestar for Ford? I don't remember an indirect system on any other Ford vehicle other than a minivan).

 

The best method that I can think of would be to over inflate the tires if you have to drive some place to fill them, let the car sit until you can take a cold reading then adjust as necessary.


The only vehicle manufacturer I know of that included a table for adjusting pressure based on temperature including operating speed is (or was) Porsche.


I'm sure there is enough information out there on the interwebz to allow you to calculate the required pressure for a given temperature when you want to fill your tires but that's just a snapshot of the tire at the time the temperature was measured and wouldn't account for every driving cycle.


Ergo, the method I suggest by over filling and adjusting when the tires are dead cold. If the gas filling the tires is reasonably dry then it wont take much. 5 psi or so over the cold inflation pressure desired (pursuant to the maximum pressure branded on the sidewall of the tire) should do the trick.


I'm curious though how often you plan to check your tires? Once a month is more than adequate for a vehicle driven on the road to account for seasonal variations except for an extreme swing in temperature.