The additive must be properly mixed with the fuel - and in the correct ratio. Calculate how much additive you need per gallon and the total to add to your vehicle's tank size. Your fuel tank should be nearly empty before you refuel. Drive to the filling station, switch off your vehicle, and add the correct amount of additive before you fill the tank. This will allow the additive and fuel to mix properly. Repeat the process when you are nearly out of fuel, as needed.