Turns out it's all physics. Here is the fuel consumption map for a 300SD.
I'm only beginning to understand the graph, so for clarity I'm quoting Arcosine from Ecomodder who explains it quite well...
Quote:
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The graph is a contour plot of a hill, the top of the hill is the best engine efficiency. The dotted line is trail that goes along the hill. This is where the engine operates on level road. BMEP is how much the engine is loaded, the bottom is no load, top full load at any given RPM. KW is the engine power output, the upper graph. The curved lines going up to the left are engine power isolines. So at 2500 RPM at the dotted line, the car needs 20 KW on level road and uses 300 g/kwh so 300*20 = 6 kg/hr of fuel. Diesel is .8 kg/L so it burns 7.5 L per hour, so that 2 gallons per hour and if the car is going 60 mph at 2500 rpm, that would be 30 mpg.
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All the usual suspects change the road load line. Headwind moves it down, tailwind moves it up. Uphill moves it down, downhill moves it up. Under inflated tires moves it down, over inflated tires moves it up. Decreasing coefficient of drag moves it up, increasing coefficient of drag moves it down. Decreasing weight, moves it up, while increasing weight moves it down. Drafting a semi moves it up, while riding the brakes moves it down. Swapping a manual transmission would move it down and downshifting would move it down.
This is a steady-state graph, so during periods of acceleration, the graph doesn't apply since it is not steady-state. Stop and go is deadly on fuel usage, as is idling, and so forth.