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. To unsubscribe or to change to a daily Digest, please go to http://www.airstream.net/vaclist/listoffice.html If replying back to this message, please delete all the unnecessary original text from your reply.
