Simplify the following text for a Grade 6 native English-speaking person.
Original: In May 1980 after finding that they knew the tires were defective , the National Highway Traffic Safety Administration fines Firestone $ 500,000 USD , which at that time was the largest fine imposed on any U.S. corporation and the largest civil penalty imposed since passage of the 1966 National Traffic and Motor Vehicle Act .
Simplified: In May 1980 NHTSA fined Firestone $ 500,000 USD . At that time was the largest fine imposed on any U.S. corporation .