1955 Chevrolet trucks changed from 6v to 12v when the models changed during that year. GMC trucks stayed standard 6v through all of 1955 (even for the new 1955 Blue Chip trucks).

1955 Chevrolet cars changed to 12v at the beginning of the 1955 model year.

However, this 1955 Thunderbird website says that 1955 6 cylinder Chevrolet cars were 6v and V-8 Chevrolet cars were 12v (while the V-8 Thunderbird in 1955 was 6v).

The 1955 Chevrolet Passenger Car Shop Manual states that a 12v electrical generating system was used on all model cars.