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Originally Posted by eleven24
I don't think BMW utilizes a TPS inside the rim. I think the tire pressure is monitored through the DSC system I'm pretty sure of this, but if someone could verify...
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Yup. The ABS/DSC sensors monitor tyre rotation. If one tyre is rotating slower than the other (which would happen if one tyre has less air than another), then the pressure warning 'dings' to alert you that one or more tyres have lost pressure.
What it wont do is take into account normal air pressure losses that occur with every tyre. That, coupled with the fact that the tyre is a runflat, you ordinarally won't be able to visually determine when the tyres are getting low in air. That's why you should carry a pressure guage in the car to periodically check the pressures (because the ones at petrol stations are usually inaccurate).
I check my pressure whenever I fill up and always 'overinflate' by a litte. Then later in the evening after they've had a chance to cool off, I use my guage to deflate them to the correct pressure. Also, keep in mind that winter tyres should be inflated a little higher than summer tyres because air contracts when it's cold. If you're using your guage outside on the street with tyres that have been sitting for a while then it shouldn't be a problem to put the normal pressure in. If you're measuring your tyres in your garage which is say 15 C when it's -10 C outside, then there tyres will actually 'lose' pressure outside as the air contracts due to the cold. So it's wise to leave a couple of extra PSI to counteract this.