I averaged 25mpg on an all highway trip during the summer but it was very boring and cars passed me the entire time...I've found it gets the best gas mileage going 60-65mph in 5th gear at absolute minimum throttle. Don't try and keep your speed on the uphill and just let it go on the downhill. Unfortunately, in the city, I've found that you can't just drive like a granny and get good gas mileage...the stall is too high and it will just waste gas. Give it a good amount gas until you get up to speed and put it in 5th at all speeds above 30mph to maintain speed. I wish I got good gas mileage again, but I'm plagued with a check engine light as usual.

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2006 X5 3.0i: Jet Black with Truffle Brown Interior, Auto, Navi, Xenon, Premium
2007 335xi: Black Sapphire Metallic with Black Interior, Manual, Xenon, Premium, Sport, Cold Weather