To make it even more confusing, people in metric countries could use metric miles. Unfortunately, depending on context, that is either 1500 m (swimming) or 1600 m (running), except in Britain (you Brits really have to have things your own way, don't you?) where it is 1500 m in running as well. For automotive purposes, I would choose 1600 m, since that's closest to the statute mile. I have no idea what the abbreviation for a metric mile is, but I supposed "mmi" would work.
Or the British Empire and all its offspring (including the USA) could pull their heads out of their asses and go completely metric.
On a side note, because mpg and l/100km are reciprocal measures, distance per volume vs. volume per distance respectively, the conversion between the two is the same both ways, y = 235/x.
For example, 30.0 mpg = 235/30.0 = 7.83 l/100km, and 8.00 l/100km = 235/8.00 = 29.4 mpg to three digits precision (the same precision as the formula).