Almost every mercedes has that on the gas filler door, I believe it's for minimizing tire wear when you drive faster... I usually put the tire pressure in between the under 100 and over 100.

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2006 BMW X5 4.8is Lemans Blue/Beige/Dark Maple
-Nav, Comfort Seats, Heated Steering Wheel,
Rear Airbags, AUX Input
2009 BMW M3 Space Grey/Black/Carbon
-all options