Tax cars by a formula based on weight and miles driven. That's the concept we're explaining in our second series, where we're joined by Felix Leach, Associate Professor of Engineering Science at the University of Oxford. He and Nick are the authors of a new book - Critical Mass: The One Thing You Need to Know About Green Cars. It unveils a radical new concept in sustainable motoring, and how to harmonise it with governments' need to raise revenue. But how would it really work? Oliver's on hand to put Nick and Felix through their paces.