The slower you go (within reason) the better your mileage will be.

I just looked back at the manual I have for a 380 SEL which includes a fuel consumption graph. The line starts on a relatively flat path at around 40 km/hr with fuel consumption at 8.5 L/100km This indicates that the best MPG for this car would be approx. 30 MPG @ 25 MPH.
As the speed is increased, so is the fuel consumption, and it increases exponentially.
The graph shows fuel mileage to be 24 MPG @ 55 MPH, and 9.4 MPG @ 125 MPH.

There is no "flat spot" in the graph where fuel consumption is the same or lower with increased speed.