Of course, even that isn't an apples-to-apples comparison because a gallon of diesel fuel contains more potential energy than a gallon of gasoline. Add in the fact that internal combustion engines running on diesel fuel are more efficient than their gas-powered counterparts, and diesel starts making even more sense.
Still, it's not all rosy for diesel engines in America. There are many ways to measure emissions, and in some cases, diesels aren't so clean. Specifically, oil-burners exhaust higher levels of nitrogen oxides and sulfur oxides along with particulate matter (soot) in the form of unburned carbon compounds. All of these issues can be mitigated somewhat, but that costs money. Still, diesel (including biodiesel) is an alternative that is looking more and more practical as time marches on.
[Source: Energy Information Administration via The Car Connection]