locking gas caps are not really the answer, every seen the damage done by some one prying the cap off? a better solution is to get one of the spring wound coils, they have a large opening on one end and tapper down to about 1/4" on the other, you screw it in the fuel tank filler pipe, and push it down a few inches so you can get the pump spout in the filler pipe, once in almost impossible to get them out, does not restrict adding fuel , and you don't have to worry about losing a key, they have them on ebay for around $7