But before you shell out about $15 on a piece of plastic with a key, think it over:
Do you really need a locking gas cap?
If you have a fuel door on your car, wouldn't that be enough of a deterrent? Well, maybe. The serious criminal may just resort to prying the door open with a screwdriver, but on the other hand, gas theft is a crime of opportunity and sometimes a little bit of an obstacle can be observed as a big roadblock.
SUV and truck owners should note that they are prime targets for two reasons. The tanks are larger and the height of the tank above the ground makes it easier to siphon. Even so, sedan owners shouldn't become complacent since some of the thieves use battery-powered pumps.
Are you ready for the inconvenience?
The need to carry and access the key each time you're fueling can be a pain--you must decide whether to provide a key for every driver or to leave the key in the vehicle. Where to keep the key can be troublesome--does it go on an already overpopulated keychain, or somewhere else? Even then, fiddling with a key in the rain or the cold will make you wonder whether it is worth the trouble.
What are the alternatives?
The usual precautions may help you protect your gas better than a locking cap. Park in a well-lit area, and position the car so any lurkers will be exposed to passers-by. Or, you could go radically simple, and simply keep your gas tank less than half full, to limit your losses.
So what do you do if you misplace the key to the locking gas cap?
You will need a pair of large channel-lock pliers. The jaws on this tool need to open at least 3 to 4 inches. Seasoned repair people call these water pump pliers. Place the jaws around the outer rim of the locking cap and apply pressure against the plastic while moving the pliers' handles counterclockwise. This works well on locking caps made of plastic. If you are dealing with a chrome-plated metal cap, you can try the same technique but you are probably going to destroy the cap in the process, which actually might happen on the plastic cap as well.