You might not think the basic fuel cap on many vehicles would present a problem, but it does. To start, if you don't tighten your fuel cap all the way on, your check engine light pops on. Then there are a number of vehicles who don't have the gas cap on a strap which means you could lose it. Finally, you run into the problem of someone syphoning your gas tank due to the high prices at the pump.
Ford has a solution to these problems with the Easy Fuel Capless System. Autobytel's Road Test Editor, Benjamin Hunting, gives you an overview of the system.