The year was 1970, although if you wanted to drive to Indiana, Standard Oil gasoline stations there had been selling a hi-test grade of unleaded gasoline since 1922. Sunoco stations everywhere had also been selling a high-octane unleaded brand that they dyed a blue color since the 1940's.
But, the real big transition year was 1974. Though there was clean air legislation also driving change, the entire country was scrambling at that time to offer unleaded fuel because the 1975 model year automobiles (which came out in September of 1974) mostly had catalytic converters for the first time and lead destroyed the converters. To make sure this didn't happen, a different nozzle size was invented for unleaded pumps and auto gasoline tanks, and it is still with us to this day. Unleaded fuel was technically available in California and many other major metro areas between 1970 to 1974, but refiners didn't make a lot of large commercial batches because there was nobody to sell it to quite yet. During this horrible fuel crossover period, people found out quickly that if you tried to save the planet by putting unleaded fuel in leaded cars that needed it, you would get loud pinging, knocking and after-run when you turned the ignition off. Talk to car owners from back then and they all have stories of turning their car ignition off and the car chugging and spitting for another 30 seconds afterwards, trying to digest the unleaded fuel in a car that wasn't designed for it. Generally, you only did that once, and went back to leaded fuel. That is until you bought a 1975 model car or later.
