Our young Leerburg puppy has developed a habit of wanting to chase cars when I walk it. What should I do?

Ed,

Our great puppy, Xanda vom Leerburg (born January 16, 2000; call name "Rebound"), is progressing beautifully. She is almost five months old and has settled in as a full-fledged member of the family (which includes my wife, three children, our male yellow lab and me).

I would like your thoughts on a particularly disturbing and dangerous habit that she has exhibited. During our walks in the neighborhood, she literally lunges at moving cars. In response, I shorten way up on the leash and tell her "no." (Often, I walk her with our lab, which does not give the cars a second glance, and has never chased them.) Needless to say, the puppy's attitude toward cars turns otherwise very enjoyable walks into quite a job. Any suggestions?

Thanks for your time.
Bob