'61 chains' describes a discrete unit of length and is therefore singular, so 61 chains is 0.7625 miles.
Having sought "higher authority" (a teacher of English), I am advised that the correct word to be used in this case is indeed:- "are".
Teaches English where, exactly? Sidmouth College?
The subjectS are "61 chains", not anything else, so the use of "are" is grammatically correct.
At this point I should make it clear that I didn't actually say that the use of "are" is incorrect.
However, as I read it the subject is '61 chains' and not just 'chains'; it is therefore correct to say that there
are 80 chains (subject is 'chain') in one mile, and that 80 chains (subject is '80 chains')
is one mile.
Your explanation is akin to saying "eleven men is playing for Arsenal this weekend", because 11 men make up a discrete unit of people, ie: a football team.
The teacher quoted a similar (reverse), but also very common incorrect usage: "England are losing one nil", this should be "England is losing one nil", since "England" does indeed refer to a single entity in this case - the team. To use "are", the response should be something like "the England players are losing one-nil".
You can go quickly mad trying to work out the plurals of mass nouns, count nouns and collective nouns: why, for example, is it always 'fruit and vegetables'?
And did Elvis Costello get this wrong, or could he simply not make his mind up?
Oliver's Army is here to stay, Oliver's army are on their way...
You could always try Googling "metonymic merging of grammatical number". If nothing else it will keep you out of my hair for the next few hours
PS: I might point out that your use of "mile
S" is incorrect as there is actually less than one mile - but I won't !
Pfft.
http://www.worldwidewords.org/qa/qa-non2.htm