Mileage for restored cars is irrelevant. For unrestored cars, it does make a difference. If you had a 8,000 mile Daytona it would go for much more than the prevailing market.
I think this trend is true for modern cars as well, but they haven't been restored yet. As F40's will start to into that zone where they will be restored by owners, I think the market for F40's will change slowly. One thing is problematic, of course, and that is that the F40 was already a speculation car before the first one was delivered. So there are many that have not accumulated mileage, and this has made the cars that have actually been used stand out.
For regular production Ferraris there is a change in the air - here in Europe Ferrari's are used much more than 15 years ago. Finding a 360 Modena or 550 Maranello with less than 40,000kms is not easy. 458's that do 20k a year seem to be abundant. For the US, this doesn't seem to happen as much.
For me, I would not be worried by a 80k kms (50k miles) 550 or 360. Is it high mileage? For me, no. It comes down to the quality of maintenance and how the owner looked after the car.
A 50k 308 from 1976? Well my Boxer has 56k miles and is from 1974. Is it high mileage? Heck no! It is an average of only 1,300 mile per year for my Boxer. My car drove only 1,000 miles between 1988 and 2008! You are now firmly in classic territory, though, and if you would want to buy a car like that and use it, you need to invest in restoring the car at least partially, particularly mechanically. This is very expensive, and 308's tend to still be seen as the cheap Ferrari. But times are changing. The vetroresina cars are now starting to command very high prices, even at high mileage. Looks like they're moving up and the 348 will be the next entry-level.
So to answer your question Boxer - I don't think this discussion will ever go away and I think there is no rule that you can work out that will determine what is high mileage. It depends on the model and how it is regarded in the market, and the latter changes over time (as the mileages do as well), but they also change regionally. It also is a personal thing - I am much less fussy about mileage than other owners I know.
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