I've consistently had 21 to 23 in town which is majority of my driving. My last 200 mile trip to the mountains and back was just over 28. I have about 16,000 miles on my 2011 now (May 2011 purchase). My diesel prices have varied from a low of $3.70 a gallon to $4.29 now (winter months higher...summer that better price).

I do also find the AC a big drag on performance and when temps dropped here and I no longer need AC (I just switch off the AC button after hitting Auto on the climate control) the X has nice throttle response and feels quicker.

I've not regretted the purchase. Always seems to have plenty of grunt to get the job done and put a smile on my face. As another poster said if you go to the 50i mileage is probably not your biggest concern. I think where the diesel shines is the balance between mileage and performance. My .02 cents.

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2007 328i Sport, Monaco Blue
2011 X35D - sold -Space Grey
2004 X5 4.4i - sold - Kalahari Beige
2001 X5 3.0i - sold - Pearl Beige
2006 325i Sport, Sonora Beige
2000 323i - sold - Sienna Red