EPA emissions figures
Here's a table showing emissions from biodiesel as compared to petroleum diesel, compiled by the U.S. Environmental Protection Agency (EPA) from a survey of the large body of biodiesel emissions studies.
What are the potential drawbacks of biodiesel?
We believe that the advantages of using biodiesel far outweigh the potential problems, but we can imagine smart readers thinking, "there's got to be a catch". Here are the two main environmental issues with biodiesel; we believe both are quite solvable.
Although biodiesel emissions are mostly much cleaner than for other fuels (see the EPA information above), one emission component is a bit higher: NOx. This can be addressed with catalytic converters and well-designed and tuned engines.
Another issue is the amount of land and other agricultural resources needed for making source oil. To make enough biodiesel to totally replace all fuel used for heating and transportation in the United States, according to Wikipedia, "it would require twice the land area of the US to be devoted to soybean production..." There are significantly better crops for oil production than soybeans (rapeseed/canola yields more than twice as many gallons per acre, algae much more), and research into improving oil yields continues. Very promising work is being done with cultivating very-high-yield algae. And conservation plays an important role in any energy plan.
How much land does it take to produce a gallon of biodiesel? To drive a mile?
The very rough estimates below for usable oil yields per acre are derived from a Wikipedia article. There's actually quite a range for each oil source type.
Again, we're using our 44 mpg example. Farm equipment, buses, trucks, etc. would all use more gallons per mile.
Thanks to Rick Banks for checking and correcting these figures.
One square mile contains 640 acres.
There's lots of interesting and useful information in the National Biodiesel Board's collection of fact sheets.