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I haven't done much testing of my current car on gravel but have some numbers for my 2001 Explorer.
Basically those numbers were all over the place depending on conditions. When the gravel was soft yet smooth it didn't seem to matter what speed I went, I would burn around 15-18 l/100km (15-13mpg) on hard roads with barely any gravel I would get almost the same as paved surfaces in the 10-12 l/100km range (19-23mpg) but on corrugated washboard I had widely varying averages from 15-32 l/100km (15-7mpg). Some of the worst averages were when I slowed down too much to "float" over the washboard and continued at that speed for science, probably contributing to blowing another set of shocks.
So my advice, drive however is the most comfortable for you on gravel. In the thick stuff there is so much rolling resistance that wind resistance is a relatively minor factor so just find a speed you are comfortable with, and in the rough stuff find the speed that works with your suspension.
Another thing to consider is how many gallons of fuel you need to save to pay for a windshield. It's OK to lose some momentum when meeting another vehicle...
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