Recently my '08 Prius was struck hard from the rear while
stopped at a traffic light. It was propelled forward into the
car ahead with resulting damage at both front and rear. An
unpleasant story that I won't go into further.
Despite the damage, the car was drivable with no apparent
mechanical or electrical issues.
One not surprising result of the collision was that the plastic
cover that wraps around the rear bumper and lower body was
shoved forward. The result is that the forward facing ends
popped free of their restraints and stick out some 4 inches.
They form anti-aero scoops at the worst possible place, at the
rear where best aero practice is to smoothly lead air separated
by the passage of the car to rejoin with as little turbulence as
possible.
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What surprised me was how great the apparent effect was.
Given thatair temps were ~70 degF, I was familiar with the
roads, traffic was light, the car was fully warmed up, I would
expect about 55 MPG on the 8 mile trip home with PSL of
35 MPH. My ScanGuage registered only 44 MPG.
That's a 20% hit from what looks like a small disturbance.
There is the possibility that the alignment has been screwed up
which would contribute to the MPG reduction. If so, it wasn't
noticeable... but yes, it could be there. (I'm asking for a four
wheel alignment check/realignment as part of the repair.)
The message here is Toyota did a great job with the aero tweeks,
and on the Prius even small changes at critical locations are going
to seriously hurt FE/MPGs.