The same thing happens, often, on my favorite bike trail. Heading out, it's a very gentle downhill with about a 10 mph headwind. I feel like hot stuff--the wind keeps me cool, and the gradual downhill lets me keep close to a 20 mph average despite the headwind. Then, when I turn around, it's 12 miles of persistent uphill, with little or no sensation of air movement--the tailwind is about the same as my average speed. I arrive back at my car chastened and dehydrated.