Hi there

This is my first post here. Like in a lot of "first posts", I will probably sound stupid in this one.

Anyway, I've asked this question to other SEOs and none of them gave me a clear answer.

Here's the thing: If, in an ad group, I'm already advertising for the word, let's say, "cars", does it make any sense for me to buy more specific words like "new cars", "second hand cars", "cars for sale" and so on?

What I'd like to be sure is if it won't make a difference to buy a more specific keyword when I already advertise for a generic one. And, if I am advertising for both "cars" and "new cars", and someone enters the query "new cars" and clicks on my ad, which keyword will Google/Yahoo give the click credit? "cars" or "new cars"?

Thanks for the help!