(Update)
The developers responded to my support request and resolved the issue with the editor. It works as intended now. Excellent and responsive support!
Unfortunately, one cannot create a conversion with reciprocal units, like L/100km to MPG. The SI value needs to be directly proportional to the Unit, not inversely proportional. Perhaps add an exponent constant in the formula to create a reciprocal?
Regardless, I increased my rating.
(Old review)
There doesn't seem to be a way to add additional unit conversions and when editing an existing, even just changing the name, it resolves to zero. The only way to fix it is to reset all units to default. That being said most of the units one would need are there. It would be nice if there was also a fuel economy conversion, but I created my own using the solver.