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30 - 32 is correct, the 51 on the sidewall is the maximum inflation pressure that should not be exceeded.
What vehicle YMM are these mounted on?
The TPMS system will come on at 26 psi and your normal system pressure is 32 psi. As these are low profile you can run a little more pressure up to 40 psi cold to keep them from bottoming on rough roads but the ride will be harsher. Run the pressure on the door post and you will be good.
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