[RedDevil]

Whenever there's a side wind I see my instant fuel economy rise whenever I pass something that blocks it, like a group of trees, buildings or a noise barrier.
This happens even when the wind is supposed to come from behind, so its relative speed in the wind shadow would be higher than out in the open. More friction just from hitting my car at an angle.

Likewise, if I come closer to a semi the turbulence in the wake may increase fuel consumption while on average the wake moves in the right direction; but the messy pattern kills the orderly flow over the car resulting in slightly higher friction.
When following a semi I may not have to adjust my CC settings for miles on end, because the added resistance slows my car a bit when I come within 40 meters or so. In economy mode the CC allows variations up to 3 km/h from the set speed before adjusting the throttle, which makes this possible.
Apparently 50 to 60 meters is the sweet spot where the wake still has some effect but the turbulence isn't strong enough to mess up the aerodynamics.