According to the National Highway Traffic Safety Admin, in their report dated Jan 2006 about "Vehicle Survivability and Travel Mileage Schedules", which you can download here:
http://www-nrd.nhtsa.dot.gov/cats/listp ... By=DocType
for light trucks over 36 years old only 2.9% remain registered and driven on the roads. Go to page 25 for one graph on survivability rates, there are others as well.
No breakdown of brands of trucks is given, nor do they have data over 36 years old. Seeing as how all of the bumps are at least 40 years old now I would assume that survivor rates are lower that 2.9%, but I'll use that number to work with.
According to this website Ford made 2,958,575 pickups of all types between 1967 and 1972.
If only 2.9% survive as registered and driven then about 85,799 are left in drivable condition.
Probably less.
If we divide that up between the 50 states then each state has 1,715 bumpsides.
Guessing that most of them are in dryer climate states, but maybe not.
So not many of the old timers are still out there. How few does it have to get before they become priceless collector objects?
