The car tax rates are getting more stupid by the year IMO, if "emissions" are the basis criteria.
You have two cars that are less than 5 years old and would be £100 a year tax band . If they both cost more than £40k new, then that is trebled up until 5 years. Your emissions for both would be, lets say, 250g/CO2/Km. £600
You have two cars registered before 2001, with combined emissions of 600g/CO2/Km, you pay £410 a year.
The above is a rough estimate, so don't shoot me, the point is you will pay less road tax for old polluting cars than newer less polluting ones, when our wonderful Government is supposed to base it on CO2 emissions!
So effectively, your road tax is "means tested", not based on emissions. If you can afford expensive cars (although £40k is not an expensive car anymore), you pay sunshine. If you run around in ratty old cars, you must be poor so we won't come after you.