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It seems most rap songs are between 75 and 90 bpm. It seems this range is determined by the natural speed at which people speak: a slower tempo would have the rapper speaking abnormally slowly, and a faster tempo would have the rapper speaking abnormally quickly.
Adjusting the tempo may be one way to adjust the mood of the song. I took Eminem's 91bpm "Guilty Conscience" and slowed it down to match Jamiroquai's 67.75bpm "Everyday" and the result was really interesting IMO. It was a lot more mellow than most rap songs, but in an interesting way.