I have found this comment on Answers.Yahoo:
Why do some football clubs like Racing Club and Racing Santander have the word, "Racing" in their names?
They were a racing club?
The club’s origins date back more than a century to a meeting that took place at the Buenos Aires Mercado de Hacienda on 25 March 1903. It was there that officials from two of the district’s rival clubs – Foot Ball Club Barracas and Colorados Unidos – sat down to discuss a possible merger. With neither club long established, the decision was taken to put aside their differences and join forces. After a call was made to give the new club “an original, distinctive and appealing” name, one of those present, Germán Vidaillac, showed his fellow officials a French automobile magazine called Racing. There and then, the decision was taken to go with this name, and Racing Club was founded.