It seems really difficult to quantify what makes an old car a classic. Is it character? How popular the car was originally? How good it is to drive? How many are left on the road?
I started thinking about this the other day when I was out in my daily car. It is already 15 years old, absolute mint condition, with only 21k miles on the clock. Due to the low mileage (and the fact I only do about 5k a year) it should easily last another 10 years, at which point I could have a 25 year old car in mint condition (if I look after it). The only trouble is its a Fiat Punto, and try as I might, I can't ever imagine a Punto being a classic car!
So why is it some cars become classics and others don't?