563 was my best on an all freeway trip through the mountains from PA to Toronto with AC going an wife and kids and luggage. A can of bg44k in the tank, new tires running 38psi and cruise @ 70mph.

Driving style is the biggest factor in MPG. Next is tire pressure (& type to an extent). Next is weight. Everything else is just minor friction losses that are used to get the vehicle moving.

And of course - your emission system should be functioning normally.

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2006 X5 3.0L - Schwarz Black/Truffle Brown
Stock
But I keep it clean