I hate mathematics but stick with me on this one. The naming system for the 456GT follows the traditional Ferrari way, giving the displacement of one cylinder of the V12. So, with each cylinder displacing 456cc, you get a V12 of about 5.5 liters. As another example, the 250GT had a three liter V12, with each cylinder displacing 250cc. By the time they developed the 456GT Ferrari probably knew they'd be using the engine for the 550 Maranello and reserved the name for that model. The earlier system always assumed a V12 engine would be used but as the company moved forward with V8's and sixes they had to adjust the naming procedures to avoid confusion. Under the old system the 308 would have been called the 375, the same name as an earlier and much different Ferrari model. Hope that helps you understand the names of older Ferraris; I used to think they were just random numbers or internal type designations from the company.