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I have a 2006 X3 steptronic, and get 17-18 suburban driving, and 23-24 highway, which sounds the same as you. The highways around me move along about 70. I keep it in sport mode anytime I'm under 45 mph, but rarely manually shift. I suppose it's how you are defining "city" driving......sitting in traffic jams, or crawling stop and go will kill you.
I find the octane level of the gas matters for mileage....go with 91-93 octane always.
I don't have my manual in front of me, but you should be able to go a long time before you change the plugs.
Considering the size, weight, and performance, I think the mileage is quite good. Not so good if you commute in heavy slow traffic. I don't know what the EPA definition of city driving is, but it's probably more like 5am on a Sunday, rather than 5pm Friday.
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