If you accidentally put regular unleaded gasoline in your car that requires premium fuel, the good news is, it's unlikely to cause any damage. Your car's engine will run a little less efficiently and you may notice a loss of power and acceleration. You might also see slightly lower gas mileage. If you continue to use regular gas, long-term damage is possible but unlikely. The reason has to do with the octane rating of premium fuel.
The higher octane rating of premium fuel means it can withstand higher compression before detonating. That's important because when gasoline ignites too early in the combustion cycle, it can ping or knock. Knocking can lead to engine damage over time.
Detonation occurs when hot spots within the cylinder reach their self-ignition temperature and start burning before the spark plug fires. When this happens, pressure waves collide, creating a knocking noise. Parts of the air/fuel mixture can actually start burning too early - way before the spark plug fires - causing an explosion instead of controlled combustion. Parts of the air/fuel mixture can start burning too early - way before the spark plug fires - causing an explosion instead of controlled combustion
So while it's not ideal to fill up with regular unleaded gasoline if your car requires premium, it won't ruin your engine – as long as you don't make a habit of it.