Quote Originally Posted by PetrolDave View Post
I think you're misunderstanding how indirect TPMS works - it doesn't have pressure sensors so it cannot measure/store any pressure values.

It measures the rotational speed of each wheel using the ABS sensors, and with some weird software compares that, with tolerances, against values of rotational speed for each wheel that get stored after you use the TPMS reset button/menu.

When you change wheel sizes or pressures all you have to do is set all the tyres to the correct pressures and reset TPMS. Since you can't see the pressures (as they're not measured) if you're unsure of the set values being used by indirect TPMS then just assume they're wrong and reset TPMS yourself when you know the pressures ARE correct.
Thanks for the reply PetrolDave. That makes complete sense, unlike what I was thinking. I knew it measured rotational speed (especially differences between tires) but had believed wrongly that it then used that data to calculate (estimate) a corresponding pressure to match to a stored value. I was too hung up on the word PRESSURE in tire pressure monitor system.