Some of the people laughing at "speed holes" are showing a degree of ignorance of high-level import drag racing. There are tons of VERY fast (think 8-10 second 1/4 mile with FWD) cars out there with so called "speed holes" in the rear bumper. I'd have to imagine that it's a car-by-car thing though. Many cars would probably see losses.
GO-AUTOWORKS.COM Rear Diffusers, Splitters, Front Ends and Carbon components
from the thread below
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on the older civics, air would catch on the rear bumper since it was open to the air moving underneath the car. Putting holes in the rear bumper would alleviate this problem. Of course a full diffuser would work better but that requires some decent skills with your hands and machinery.
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rear bumper diffuser - 8th Generation Honda Civic Forum
another thread on the topic:
rear bumper holes? - Honda-Tech
There are certainly better solutions, but for CHEAP and within certain rules, it does have benefit.