I worked at a Shell station mid 70s when unleaded had just come in.
I don't remember the capacities, but there were 3 tanks, originally 2 for Regular and 1 for Super Shell (premium).
When the Unleaded fuel started it was 1 Regular, 1 Unleaded, and 1 Super Shell.
Because of the compartment size in the trucks oftentimes all the Regular wouldn't fit in the 1 Regular tank. So we'd just hand the driver a Five Dollar bill and he'd dump the remainder in the Super Shell tank, thus making the Super Shell not so Super.........