Tires: auto tires, european common market, auto tires

Question
What is the single letter such as "E", "D" on an auto tire
stand for?

Answer
Willia,

Letters do not appear by themselves.  They are usually accompanied by something else that adds context.

For example: Load Range "E", which indicates the load carrying capacity rating for that size.

or e4, which indicates a European common market approval.

There are many others.

So in order to answer your question, I will need to know the context.  In other words, what else is there?  Is it part of a phrase, or are there numbers as part of the grouping, etc.