1975 was the first year that leaded gas was started for cars. Anything newer than that should have harder seats than the '74 and earlier leaded cars. I know that 75 is the first year of the catalytic converters for cars.
I think trucks were later to be converted like 79 if I remember correctly. The 78 Lil Red Expresses didn't have much pollution controls, but 79 did finally get them. We had a 79 Warlock II that was an emissions unleaded vehicle.
