[VAC] Tire sizes

["Dr. Gerald N. Johnson, electrical engineer" <geraldj@xxxxxxxx>]

I went to the nearby university library today and found the standards of
the Tire and Rim Association.

In their 1981 handbook they said that a 7.75-15 ST load range C (6 ply
rating) can carry a load of 1680 pounds when inflated to 50 PSI. That
tire's design section which when new is 7.75" wide and 27.24" diameter
on a 5.5" wide rim. The maximum grown tire (as it stretches with age) is
8.49" wide and 28.70" diameter. The vehicle is to be designed to clear
that maximum grown tire.

When I rolled my Caravel spare from the truck to the garage afterwards,
I found its a 7.75-15 and it says "replaces 6.70-15". 8 ply rating.

The more common F78-15 ST is rated by the Tire and Rim Association at
1710 pounds for load range C at 50 PSI. The F78-15 ST on a 5.50" wide
rim has a design section width of 7.75" and diameter of 27.30". The
maximum grown tire is 8.60" wide and 28.78" diameter. A tad snugger fit.

I did not find 7.00-15 ST in this book. I didn't look at the latest to
see what the current dimensions might be, especially in a radial ST.
Probably should have.

One F78-15 ST load range D, that I saw at a tire store Friday said it
was rated at 1860 pounds load. I don't know at what pressure.

My '68 Airstream owner's manual specifies 7.75-15 at 45 PSI for Caravel,
7.00-15 8 ply at 65 PSI for other single axle trailers and 7.00-15 6 ply
at 50 psi for tandem axle Airstreams. A 6 ply rating (load range C) tire
should be appropriate for 45 PSI operation. There might more specifics
but they are on the infamous removed page 109 of the manual.

Gerald J.