Editorial Comment:

An article in Wikipedia makes the following claim about the letter preceding the rim diameter:

" {it is a}n optional letter indicating construction of the fabric carcass of the tire:

B: bias belt (where the sidewalls are the same material as the tread, leading to a rigid ride)
D: diagonal
R: radial"

In all cases the letter refers to the construction of the tire.

What else can I tell from what you've said? The second tire is likely much older than the one with an "R", since "D" is very rarely used these days.

It is generally not considered to be a good idea to mix tires of radial construction with those of non-radial, especially on the same axle, because of the differing stiffness of the sidewalls and the handling characteristics. I'd predict that, at the very least, the two tires would wear at considerably different rates even if they are only used on a trailer.

The "ST" at the beginning stands for "Special Trailer" and both these tires were designed for trailer use, but apart from that, they still should not be mixed. If you are going to replace one, look for another "R" or radial tire, since that is going to be easier to find.


TG