From their website:
The Mercedes E300D is the only limousine of its kind in Texas. We won't have to compromise our conscience while riding in this vehicle because it combines luxury with ecological responsibility. The engine runs on biodiesel, which is made from vegetable oil. Biodiesel is the only alternative fuel to have fully completed the Clean Air Act's health effects testing requirements. To learn more about biodiesel, please visit the National Biodiesel Board.