According to the EPA’s Renewable Fuel Standards Program Regulatory Impact Analysis, released in February 2010, biodiesel from soy oil results, on average, in a 57% reduction in greenhouse gases compared to petroleum diesel, and biodiesel produced from waste grease results in an 86% reduction. See chapter 2.6 of the EPA report for more detailed information.
One particularly interesting point that the report makes concerning tailpipe emissions of powerful greenhouse gasses is the following: “For diesel highway vehicles, emissions of N2O and CH4 are almost one hundred times less than emissions from gasoline vehicles …” (pg. 493). CH4 has a GHG rating of 21 times worse than CO2, and N2O has a rating of 310. Unfortunately, they do not seem to take into account NOx’s: NO2 & NO3 which, while nowhere near as potent as N2O, are still many times more potent than carbon dioxide as a greenhouse gas and are reported to be produced more in diesel engines due to the higher temperatures of operation.
Based on this comprehensive report is can be assumed that biofuel applications that use waste vegetable oil directly, without chemical processing, result in a greenhouse gas reduction of 90-95%. This is not only due to the lack of methanol and sodium hydroxide inputs, but also because much waste vegetable oil is still thrown away by small producers and has the likely potential to end up in a landfill, decomposing anaerobically and releasing methane, a potent greenhouse gas.