In the last episode, Nick and Felix pitched their big idea - to tax cars by weight, not type of engine. They say mass is the critical environmental factor, in an electrified world of motoring. But what's the science behind it? Do the sums and formulae add up? That's the subject of this second podcast of series two.

We're joined again 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.

And Oliver's on hand, too - to put Nick and Felix through their paces.