For what it's worth, 70mph was the only speed my old conversion van would average 16mpg or above, despite its bricklike "aerodynamics", if they can really be called such...
My best tank in my Focus also came at 70mph. It's nearly identical to my normal cruising speed of 63mph, but since I'm getting somewhere faster, 70mph wins. 55 doesn't do much for my car, it's an MPG or two below 70 or 63.
'67 Mustang - out of commission after an accident
'00 Echo - DD
'11 Kia Rio - Wife's DD
'09 Harley Nightster - 48mpg and 1/4 miles in the 12's