In Chapter 9 of the book (page 138 in my copy of the book) we are told that Ponyboy's gang can "whip" Tim Shepard's gang. The reason given is that Tim Shepard is the undisputed leader of his gang and all his members do what he wants them to without question. They have discipline where Ponyboy's gang does not.
I think this gives Ponyboy's gang the edge because they are more of individuals. They are fighting for their own reasons and in their own way. They are not just doing what they are told.
You'll hear part of this given as a reason why the US should have an all-volunteer army. You want your people fighting because they want to be, not because they have to.
So Ponyboy's gang is more motivated and cares more about what they are doing. This gives them the upper hand.