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Keep in mind that typically for best economy you will want to fill your tires to high pressures to minimize rolling resistance. This works on hard surfaces (asphalt, concrete). You'll sink in gravel with high tire pressure (think about riding a bike in the stuff). Throwing gravel roads in adds a variable you will have to test and will probably require a real-time instrument such as a SG2 to tell the difference. Because only a portion of your drive is on gravel lowering your tire pressure to have better flotation may lower your overall economy, but it depends on how badly your vehicle sinks in the gravel. Try running the gravel route at the tire pressure you would run on the highway one week and compare to your improved surface routes, but then run the gravel route again another week with lower tire pressure (not extremely low, but say ~OEM-spec as opposed to max sidewall or higher) and compare again.
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