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My cold tire pressure is supposed to be 33PSI, my TPMS doesn't alert until it's below 28PSI, so there is a significant delta still.

The TREAD law does not specify when TPMS must alert, other than the tire being 'significantly underinflated'.

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The NHTSA has determined that the requirement is that TPMS alerts for 25% underinflation across any combination of tires, or 30% underinflation in a single tire.[1] Originally they wanted 20-25%, but determined that cheap TPMS sensors were not good enough to meet that threshold and did not want to impose the cost of better sensors.

[1]: https://www.nhtsa.gov/sites/nhtsa.gov/files/fmvss/TPMSfinalr...

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