Today’s discussion on the Editors’ Association of Canada listserv has brought forth an ad looking for performers with “tonnes of energy.” Hm! That would be “tons,” right? Boy, give these people 2.5 cm and they’ll take 1.6 km…
Except that there actually is a case to be made for it. It’s not the idiom, true, but on the other hand it’s not thatthe people who use “tonnes” are converting “tons” to specific “tonnes” – “A tonne of information” would be used, not “0.907 tonnes of information,” after all. So it would be more like “Give these people a centimetre and they’ll take a kilometre.” Which isn’t the idiom, but on the other hand, like “tonnes,” it is a kindred metaphor. If I say “Give him a cabin and he’ll take the ship” or “Give him a banana and he’ll take the bunch” or “Give him a card and he’ll take the whole deck,” those aren’t the standard idiom, either, but they work. Likewise, we have so many other ways of saying “tons” – oodles, buttloads, scads, trucks full, etc., and we can invent new ones as we please as long as they’re clear – that “tonnes” couldn’t really be called wrong just because it’s not the usual idiom.
Now, of course, these are special cases we’re talking about here, because they seem to be following an excessively punctilious ethic of conversion, and we know what a nuisance that can cause – deliberately round imperial figures being changed into absurdly precise metric ones – but on the other hand it’s translating an increasingly foreign cultural reference to an increasingly natural local one. So I can see both sides on this. (“I’ve looked at tonnes from both sides now…”)
Chicken that I am on these things, I’d probably switch to a different turn of phrase and avoid the issue if it came to a point of contention.