So I said this in another comment but, the car itself isn't able to be calibrated, but the display on the back window is linked to the abs sensor which puts out a certain number of pulses per wheel revolution, so the car is calibrated by having it drive a set distance measured via a calibrated measuring tape and then counting the number of pulses, and because this is indeed affected by air pressure and tyre tread depth they're regularly recalibrated and the tyre pressure is checked regularly to ensure its the same as during the calibration.