Thanks Sean, but I don't agree with your analysis:
1. Firstly, WA DOES understand reciprocal unit conversions, try "33 mpg in L/km". Neat!
2.. I know "100 km" is not a desirable way of writing a unit, but it is widely used and legislated in Europe and Australia for fuel consumption.
In my analysis, the error occurs because of incorrect operator precedence: WA divides "33 mpg in L" by "100km", which is meaningless. I can't guess why WA behaves correctly with the similar expression "33 mpg in L/km".
If WA adopted the following operator precedence, it could parse factor-units like "100km" :
1. prefix-unit (e.g. km).
2. exponentiation (e.g. km^2)
3. factor-unit (e.g. 100 km^2)
4. left-to-right division and multiplication
5. left-to-right addition and subtraction