[userbinator]

when what the user really wants to know is "How many more songs or photos can I store?"

That depends on how large they are, and that can vary widely, which is why abstracting away files (or more specifically, file size) is such a bad idea.

[slededit]

How many KM remaining depends on the terrain, traffic, driving technique, weather and a host of other factors. Nobody is expecting 100% accuracy just a rough estimate. They are already mentally doing the conversion anyways.

[userbinator]

The range of a car can't change by several orders of magnitude in normal use, unlike sizes of files.