I don't remember which calculator did this (I think GNU units), but I've seen this handled automatically with a warning saying "Warning: reciprocal conversion"
$ units
Currency exchange rates from www.timegenie.com on 2016-06-21
2926 units, 109 prefixes, 88 nonlinear units
You have: 7.8L/10km
You want: miles/gallon
reciprocal conversion
* 3.0155716
/ 0.33161209
I think it's fine for a calculator to make guesses as long as it makes it very clear that it's doing so.
