Firstly, we assume that the average is the result of an even spread (that the average length is in fact 8.5 - we only need an average of 8 for storage to be a moot point in char vs varchar).

Secondly, within SQL Server, the efficiency may be best to use char in that instance, but take in to account the code operations of trimming and I think it becomes debatable.

I agree with you on a purist level, but practically, I would go with varchar still simply for the programmers' ease.

Nice question 🙂