The original Scotty tires, General 640/15, are no longer available. You can still get that size in Firestone or B.F. Goodrich(also, I have found out that places that carry farm implement and specialized tires usually can get you this size), however many owners are choosing to go to a modern tire/size. Modern replacement size is F78-15ST (special trailer). Metric size is 205/75D15. Whatever you choose, go with a bias (either 4 or 6) trailer specific tire and not a car radial. This helps prevent trailer sway. This can also apply to alot of vintage campers.

Later Scottys (1979 LITE models, at least) used A78x13" tires.

* The diameter for a 6.50 x 15 tire is +/- 28.5",  the cross section(width across the widest part) is +/- 7", with the tread being about 4.45" wide.  So, those are the parameters of 'suggested' tire.

This is a Tire Size Calculator page from Grizz.... LINK

"I came across some information about tires.  It concerns the age of a tire.  Even if a tire is not run on the ground, it still ages.  Tires in excess of 6 years are not recommended.  A tire's week and year of manufacture can be found in the series of numbers on the tire. The last four digits in the series are the ones about when manufactured.  For example, the number 4107 means that the tire was made the 41st week of 2007.  Tires made before the year 2000 have three digits, so they are definitely too old to use.  It is also good to check these numbers when buying new tires.  Cases have been reported that tires have been sold as "new" because they had not been on a vehicle, but in checking the date of manufacture were found to be older than evident by appearance."