Follow the plate on the door, 32/33 psi cold, as best I recall, for my 18" 255s [Conti DWS in winter, Cooper Zeon summers]. If you are running 39 psi, that and the tire type might explain it.

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2006 X5 3.0 6-spd w/Evo UUC ssk
sport/premium pkgs
born Valentine's Day, 2006.