What is Correct Modern Tire Size Equivalent to ST78-13 Trailer Tire

Question:

Recently bought 175/80 tire and wheels to replace old ST 78-13 and new tires dont fit, i.e. They rub on inner axial side. Was told here tires were same size. WTF

asked by: John

Helpful Expert Reply:

Trailer tires of the ST78-13 size have not been in common use for several decades but when they were made their actual widths were not standardized the way tire sizes are today. There were in fact 4 different widths made with this same designation; the variants had a letter code, A, B, C, or D, as a prefix to the size. A and B versions were slightly narrower than the C and D versions. An A78-13 and B78-13 tire both most closely match the modern metric size 175/80-13 while the C and D are closer to the 185/80-13 size. Since the widths of these old tires varied, even if you knew the letter code from your original tires a new tire in the correct equivalent size may not fit the same.

Another possibility is that the wheels you purchased have a different offset. Offset is any inward or outward deviation from center placement of the actual tire on the wheel. A wheel with positive offset places the tire closer to the trailer; a wheel with negative offset moves the tire further away from the trailer. If your old wheels had no offset (or negative offset) and the new ones have positive offset then this would also account for the tire's inner surface rubbing on the trailer. Perhaps the retailer who sold you the tires and wheels can confirm the offset of the wheels you purchased.

I linked an article on trailer tire sizing that may be of interest to you.

expert reply by: Adam R