A propane injection system for supplementing gasoline or diesel engines in vehicles such as automobiles and marine vessels. The invention would also be appropriate for use in turbines and jet engines by providing a secondary fuel source for improved fuel economy. A propane tank is mounted into the vehicle, such as within a cavity within the trunk of an automobile, and is connected to the air intake, exhaust, and turbo charger of the vehicle's stock engine system. A switch is used to activate the injection of propane directly into the engine cycle at two points to increase engine performance. The switch could be incorporated into an existing switch, such as the cruise control of an automobile, and may be canceled by depressing the brake pedal of the vehicle.