Re the spare wheel rule is probably a carry over from what a sports car is or was. When the World Sports Car Championship started in 1953 cars such as the production Austin Healey competed but pretty quickly factory "specials" appeared changing the character of the class. To maintain some semblance of a sports car rather than a factory prototype things such as a spare wheel, luggage capacity and windscreen height were enforced. Of course the entry of factory specials spelt the demise of production cars and the escalation of the sport from amateur to professional status. As a kid I saw the Ferrari P4 race at Longford where it was in a class of its own. It is ironic that sports car races at the time were often one horse races whereas the production sports car races were gladiatorial events.