I'd like to recommend that Liters per 100 km also be shown. This rating is gaining traction in Europe, and provides a more "accurate" comparative scale of economy (the inverse of fuel/distance).
I recognize mpg should perhaps be g/100 miles, but it we are going to use a new scale, we might as well stick with an international unit and use L/100km. The difference in the scales winds up about 2:1 regardless, so it is fairly easy to convert.
If you click the "Metric" link in the upper-left corner you'll see L/100km across the site. And if you're viewing in "US" or "UK", you'll also find the average L/100km in the lower left box under "Basis Stats".
We've had a few requests for Gallons/100 Miles, but it's not in wide use. We're planning some changes to the Basic Stats section that would let people pick their favorite, more obscure metrics for measuring fuel efficiency.
But L/100km is definitely the world standard outside the US and we use it here quite a bit if you switch to the Metric view.