Why does the narrator keep a tire iron under his seat in "Greasy Lake"?

T. C. Boyle's short story "Greasy Lake" follows the story of a group of rebellious teenagers who spend a summer evening at the titular Greasy Lake, where they can drink, smoke, and unleash their "bad boy" tendencies. When the boys arrive at their destination, they see a car parked there with a couple—who they assume to be their friend, Tony Lovett, and his girl—making out inside.
Thinking they can prank Tony, they try to sneak up on him, but discover the man within the car is not him after all. The angry man tries to attack the boys, but the narrator pulls a tire iron from his car and hits the man with it. When considering why the tire iron is stashed under the seat, the narrator simply states,

I kept [the tire iron] there because bad characters always keep tire irons under the driver's seat.

In other words, he kept it there believing it would make him seem "bad."