Replacing an old, leaky or missing gas cap will save as much as a gallon of gas every 15 days and can pay for itself in less than three months. Stant gas caps are OE style and meet or exceed all original equipment performance and safety standards. A gas/fuel cap performs three very important functions; safety of the vehicle's occupants, fuel economy and reduce emissions. Safety: Without a fuel cap, gasoline vapors will be present around your car at all times. Those vapors can be ignited by a random spark or flame and are very dangerous if you are involved in an accident or rollover. Fuel Economy: According to Sun Oil Company research, a vehicle with a missing or inoperative fuel cap will allow 176 pounds (22 gallons) of gasoline to evaporate over the period of one year. Emissions: Gasoline vapors emit hydrocarbons which can cause smog.