If the car isnu2019t running, nothing happens. If you realize youu2019ve done this before starting the car, leave it off. Have it towed to the repair shop of your choice. Theyu2019ll drain the tank, probably replace the fuel filters, maybe clean or replace the fuel lines, and youu2019re good to go.If the car is running, youu2019ve got problems. Gas will burn in a diesel, but it burns a lot hotter and the flash point is a lot lower. Fuel system has to be completely flushed, obviously. Your fuel pump will have to be replaced. You may have melted your glow plugs, which would require extensive cylinder head work. You may have washed the lubricant out of the cylinders, which could cause your piston rings to weld themselves to the cylinder walls. You may have broken your connecting rods and or valves and valve springs. The newer the car, the higher the fuel system pressure and the higher the fuel system pressure, the better the chances that you just destroyed your engine.Diesels are tough, but the diesel engines in street cars are much more highly tuned and sensitive to the flash point of the fuel than the diesels in military and construction vehicles - too much kerosene is enough to do damage (most diesel is cut with kerosene), and kerosene has a much higher flash point than gasoline.