We've heard about the big idea - taxing cars by weight and mileage, not engine type. We've looked at the science, the money, and the practicalities? But what about the politics? What could persuade governments to revolutionise the vehicle taxation system? How could this concept overcome the scepticism of the press, public, and politicians who don't like meddling in the costs of driving.

Those are the big questions we explore in this final episode of series 2.

So how would this radical new system work in practice? How much would motorists actually pay? Would petrol still be taxed, too? And could you cheat your way around it?

Those are the questions Nick and Felix set out to answer in episode three of our second series.

Their new book - Critical Mass - explores the concept of taxing cars by weight and miles driven, not by type of engine.

Oliver's here to 'stress test' their proposition, and see if their answers really stack up.

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.

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.

Nick and Oliver's special guest is John McElroy, host of the Autoline webcast and one of the world's most respected automotive commentators. There's lots to discuss. How can the US electric vehicle industry compete with China's? Will the growth of EVs actually reduce emissions? And can you cope without cobalt?

Nick and Oliver are joined by James Court, chief exec of the Electric Vehicle Association of England. They discuss - is it better for the environment if *everyone* gets an electric car? What's needed to make EVs better to own and drive than petrol/diesel vehicles? And if electric cars are cheaper to run, why do people call for subsidies?

Nick and Oliver are joined by EV expert Ade Thomas - creator of World EV Day and the Ebike Summit, and publisher of ElectricDrives. On the agenda - how do you choose the most environmentally-friendly EV to buy? What matters more - price, or range? And as software becomes ever more important, will models become obsolete more quickly?

All kinds of alternative fuels are appearing on the market - made from plants, vegetable oil, waste plastic, or synthesised. In theory they provide a third route to decarbonisation - after battery-electrics and hybrids. Alternative fuels are clean and many work with standard petrol/diesel engines. So what's the catch?

The 2035 ban is looming for petrol/diesel vehicles in the UK and Europe. But there are signs - particularly in the EU - that manufacturers are pushing back. Several major car-makers are shifting emphasis away from electric and towards hybrids - or even traditionally-fuelled engines. How will regulators respond? Are we heading for a showdown? And who will blink first?

The US has quadrupled import tariffs on Chinese electric vehicles to a whopping 100%. But only China can make battery EVs at an affordable mass-market price. So does this spell trouble for taking the carbon out of American motoring? And how will Europe resolve its own dilemma? Resist China's EVs and say goodbye to the 2035 target? Or allow a flood of cheap imports, whatever the consequences?