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      09-06-2011, 12:26 AM   #36

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Join Date: Dec 2010
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Originally Posted by XB101 View Post
So with the different monitoring system would it make a difference when one replace the RFT with regular tires? My old 2004 CTS-V actually had readings for each tire. Would be nice to know exactly if one is just a bit off.
A system that indirectly detects a low tire by monitoring for any differences in rotational speed (between tires) (which apparently is what the Canadian X3's use, but not the US X3) should be more sensitive to pressure drops in a regular tire as compared to a run-flat because of the "stiffer" run-flat sidewalls.

Either system will tell you if a tire is low, but how much of a loss it takes to trigger the warning is something that the design engineer determines.

With wheels, which contain a TPMS transmitter that sends a radio signal carrying the pressure reading, the real pressure drop in PSI is measured and the trigger level can be calibrated in real units (PSI). With an "indirect system, the car's computer receives a reading of a slightly different average rotational speed of one of the tires and interprets that speed difference as a change in the effective rolling circumference which varies with the tire air pressure.

Tires of different construction or tires of different aspect ratio or possible different brands/models, will have a different "air pressure vs. rotational speed factor". Since the rotational speed triggers the warnings, the tire pressure drop (in real air pressure) will vary. It probably will be more sensitive to small pressure drops using regular tires as compared to run-flats, but the only way to be sure is to test it.

That's why I wondered in a previous post "I'd be very interested in knowing how much pressure loss is needed..."