"What if I put diesel fuel in an automobile that required unleaded fuel only?"
"The first thing to recognize is that this scenario isn't possible in most cars. In any car manufactured in the last 25 years or so, there's a plate under the gas cap that prevents anything but the small unleaded-gasoline nozzle from fitting into the tank. "
https://auto.howstuffworks.com/fuel-efficiency/alternative-fuels/diesel-in-unleaded.htm