Our advice is simple: avoid this at all costs. However, if the worst does happen, the thicker diesel fuel will immediately begin to do battle with the gas vehicle's fuel system. The fuel pump will initially have trouble moving the denser liquid - mixed with leftover gas - through the system. The fluid that does reach the engine will likely block up the injectors, which will inevitably lead to seizing. While your vehicle can briefly run on a mixture of the two fuels, you won't get very far. Even worse, putting gasoline in a diesel engine car will cause tremendous damage since gas ignites more quickly.