BMW claims 4.7 liters/100km "combined" for the 320d here:
BMW 3 Series Coupé : Diesel engines (BMW 320d, 325d, 330d)
That translates to 50 US MPG, if I'm not mistaken, so 57.4 US MPG on a long trip sounds reasonable.
This site claims the 57.4 is definitely US MPG:
BMW 320d EfficientDynamics did 1013 miles on one tank | BMWCoop
BMW has apparently fitted low rolling resistance tires and more aerodynamic wheels to the 320d as well:
Drive of the week: BMW 320d Efficient Dynamics review | New Car Reviews - Times Online
And 0-100KM in 7.9 seconds!