You really can't depend of your gas mileage when traveling through the vast nothing (it does have it's own kind of beauty). Running into a 35/40 MPH head wind can do some nasty things to your MPG. Best to be careful, take a break, hydrate and fill up at half a tank.
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Not trying to hijack but...you mentioned the beauty of the vast nothingness....and your right
And yes....when out west if ya see gas....buy gas.