One of my sons has paid $0 for fuel for almost 15 years. The answer is simple (and illegal because it can't be taxed). He drives a VW Jetta diesel and has made arrangements with several restaurants and McDonalds to collect their used deep-fry grease (WVO or waste vegetable oil). When Rudolph Diesel released his engine, it was designed to burn vegetable oil, NOT petroleum. The mileage is the same as petroleum and WVO is actually been tested as being better for the engine while maintaining almost the same cetane rating.