Re: 78 -13ST tire size
Vintage Tire Conversion Chart RADIAL REPLACEMENT GUIDE
Size Load @ 50 PSI New Size Load @ 50 PSI
SPECIAL TRAILER SIZES/LR-C
B78-13ST 1315 lb. ST175/80R13 1360 lb.
C78-13ST 1410 lb. ST185/80R13 1480 lb.
F78-14ST 1710 lb. ST205/75R14 1760 lb.
G78-14ST 1865 lb. ST215/75R14 1870 lb.
F78-15ST 1710 lb. ST205/75R15 1820 lb.
H78-15ST 2035 lb. ST225/75R15 2150 lb.
SPECIAL TRAILER SIZES/LR-D
H78-15ST 2370 lb. ST225/75R15 2540 lb.
I got that info here
http://www.cerka.ca/catalog/TIRESIZE.asp
Jeff