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In the Invester's Guide to American Convertibles and Special-Interest Automobiles 1946-1976, published in 1979, there is a formula used for how many old cars are remaining.

How the numbers are arrived at: Numbers are based on registration figures, not absolute number remaining-its a general idea of a particular car's rarity.
Next, the findings- After 8-10 years, all cars tend to take a sharp dive in survival, some at a greater rate than others. It is not uncommon for cars past their fourteenth birthday to disappear at the rate of 30 percent (or more) of those remaining per year. Plentiful cars have a way of becoming scarce in a very short period of time.

Numbers from the book - a few samples with varying production levels:
10 year-old cars in 1979 when book was published:
1969 Chevrolet Camaro Cpe/Convt -
# produced: 243,100 # remaining in '79: 121,550
1969 Chevrolet Corvette Conv't -
# produced: 16,608 - # remaining in '79: 8,304
1969 Chevrolet Corvair Conv't -
# produced: 521 - # remaining in '79: 260

20 year-old cars in 1979 when book was published:
1959 Chevrolet Impala Convt -
# produced: 69,282 - # remaining in '79: 1,100
1959 Chevrolet Corvette Convt -
# produced: 9,670 - # remaining in '79: 200
1959 Cadillac Series 62 Convt -
# produced: 11,130- # remaining in '79: 280

30 year-old cars in 1979 when book was published:
1949 Chevrolet Styleline Deluxe Convt -
# produced: 32,392 - # remaining in '79: 65
1949 Chevrolet Styleline Deluxe Wgn -
# produced: 3,400 - # remaining in '79: 6
1949 Cadillac Series 62 Convt -
# produced: 8000 - # remaining in '79: 21

The 30 year old-car sample just above gives an idea how many 30 year-old cars are left, not many.

A Possible Glitch:
There is a glitch with the formula presented in the 1979 book, and it was touched on even then. More people are preserving and collecting old cars than in the past which could effect survival rates, but even so, this pertains mainly to very desirable Early Classics and 60s Muscle Cars. These are the cars really driving the Hobby, the Auctions, and the Aftermarket companys, going as far as offering complete new bodies for the most popular cars like the 69 Camaro presented above. This assures a marque's ability to survive in greater numbers. You can probably expect most to conform to the formula.

Numbers from the Bureau of Transportation:
2.6% of the vehicles in the US are estimated to be 30 years old or older.
Per Bureau of Transportation stats, the 2006 stat for number of vehicles registered was 250,851,833.
2.6% of the above figure is 6,522,148

I've had the book as a reference for many years. I found the BOT percentage today with a two minute Google search.