I recently got 27 mpg. This was a split of 70/30 highway/around town driving, and I use Mobil diesel. My tank was just a little under the half tank mark and the trip odometer was reading 289.8 miles, she took in 10.8 gals. That works out to exactly 26.833333333 mpg. So I rounded up to say I get 27. Not too shabby for a car that weights a little over 4,000 lbs.
I say Mobil because where you buy your diesel is very important. You should buy diesel from a place that only sells to a few customers, because that means that diesel fuel is just sitting there. I buy from a station that see alot of business from truckers. This way I know that the diesel they sell is "fresh". My mechanic said that you should never try to save a few pennies by buying an off brand diesel. It will catch up with you. You should always run fresh clean diesel in your car, and check the pre-filter.

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1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member