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The increased likelihood that music heard in a car is perceived to be of higher quality is directly proportional to the speed of said car.

In other words, the faster you're driving, the better shitty music, such as R&B and the like, sounds.

Corollary to xkcd comic "Moral Relativity"
Some researchers postulate that the Speeding Car Effect is caused by the increased amount of adrenaline in the blood, allowing your body to deal with the pain of listening to such disturbing occurrences as Ke$ha.
by FMN13 July 18, 2011