Before "lead awareness" became a thing, lead was added to just about everything from paint to petrol. In 1919, researchers found out that adding lead, specifically tetraethyl lead, reduced occurrences of auto-ignition, which happens when combustion temperatures ignite the mixture too early. This caused engine knocking and, eventually, damage.Although still used by other vehicles such as off-road vehicles and farm equipment, leaded fuel is out of the passenger car category. Tanking up your ride with leaded gasoline is grounds for a $10,000 fine. Today, many new and used cars for sale at Indianapolis dealers such as Fletcher Chrysler Dodge Jeep RAM run on either unleaded gasoline or diesel.