grey,

I'm not sure where you see 40 psi recommended. Recommendations over get over 35 psi for heavily laden X5's going over 100mph / 160 km/h.

As per Owner's Manual and the sticker on door, I have mine at 32 psi in front and 35 psi in rear.

I have the sport package (staggered 20" wheels) and third row seats.

EDITED after reviewing Owner's Manual...

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2008 X5 4.8i | Wheels: Summer> style 336 20" RFT; Winter> style 214 Ferric Grey 20" RFT | Aero kit | ESS E-Flash tune | V1 | LCI Rear Tail Lights | Coding completed | Engine replaced at 90K