But surely if you don't have the key you can't start it so therefore there's no point putting petrol in? Yes, it's not called Gas it's called Petrol. Petrol is a liquid, not a gas. So calling it "Gas" is just retarded. But i digress. You should be able to get a lock set off ebay for cheap. If not, you could always take out the current fuel cap and replace with a non-lockable one.