It kills me sometimes to see people going on forums and asking questions about how to aerodynamically modify their cars. Why? Because almost inevitably they get a stream of answers that state, quite confidently and often unequivocally, that such-and-such a change will lower drag by x amount, or that a specific modification will always reduce drag, or what the dimensions of a modification should be for maximum, “guaranteed” drag reduction.
Both cars have hoods, windshields, and doors, etc.—of
course—and were tested with no mirrors. Yet the flow over the windows is
very different. Differences in the vehicle shapes, A-pillar rounding (the
truck actually has rounder pillars; the Prius has a sharp edge running along
the pillar), windshield angle (~40° from horizontal on the Prius, ~60°
on the truck), hood curvature, windshield curvature (again, perhaps
surprisingly, the Prius’ windshield is flatter than the truck’s), wheel/tire
size, wheel arch opening, ride height, door shape, cooling air inlet size and
location—all these things are different between the two vehicles. Looked
at this way, it’s incredible that anyone would expect that the same change to
both cars would have anywhere close to the same effect on the airflow over them.
|Horizontal wire traces of the A-pillar shapes of the truck (left) and Prius (right). The windshield is at the top and side window at the bottom.|
Okay, but the Prius and truck are very different
shapes. What about similar shapes? What about, say, two trucks?
|Answer: not very. Less disruption from the mirror, less separation at the A-pillar, and less turbulence in general can be seen on the newer truck, tested at the same speed, on the same road, and in similar conditions.|
Take this to heart. Stop guessing. Start testing.