Keep my Jeep on empty!
I had a locking gas cap years ago. When I left the key at home, and almost out of gas way away from home, the local auto parts store guy told me about using a screw driver to lock in the cap, and then it would turn to open. Took just about the same amount of time to open it that way as using the key.
Since then, I just don't use anything.
I know gas is expensive, and just figure I would not want to have to replace a cap if someone really wanted my gas.
End of the day, they are only going to keep the honest person out.
Todd