AT the beginning of April, the tax rules on new diesel cars changed. This only applies to cars registered from that date - those that were registered before April 1 will be unaffected.

All cars currently follow the standard first-year tax rules set in April 2017. This can vary from £0 for a zero-emissions electric car to up to £2,000 for the worst-polluting vehicles.

Then, when the car is taxed for the second time, another standard rate is introduced.

Most petrol and diesel cars have a £140 standard rate, alternatively fuelled vehicles see a £130 rate, while zero emissions don't incur any cost. The first-year rate is usually included in a new car's on-the-road price.

However, if the car you bought cost more than £40,000 at the time of registration, there's an additional rate to take into consideration - and this is applied on top of the standard rate. It's £310, regardless of the type of car - and is payable for five years from the second time that the car is taxed. That means that drivers of a high-emissions, high-price car could be paying £450, while the bare minimum that drivers of cars over £40,000 will have to pay is £310. Even if your £40,000 car is zero-emissions, you'll still pay that additional rate.

You'll have to be careful with the options box-ticking, too, if you want to avoid a higher rate of tax - even a car with a list price of under £40,000 will be eligible for the additional £310 if its price is pushed over £40,000 with any extras.

Then, when this five-year period is up, the vehicle will be taxed at the standard rate - £140, £130 or nothing.

The shake-up, which was announced in last year's Autumn Budget, concerns new diesel cars. The revision adds a tier to the table of taxation of new diesels that don't meet new Euro 6 emissions legislation and could see drivers having to pay up to £500 more.

The new rules require new diesel cars to meet real-world driving emissions (RDE) standards, meaning that vehicles will have to emit no more than 120mg of nitrogen oxides per kilometre - 1.5 times the current limit of 80mg/km - during real-world conditions. If a car fails this, it doesn't get that all-important certificate. That means there's a bump up in the cost for taxing new diesel cars, which could range from £15 to £500 and is only payable for the first year.

No cars currently on sale meet these targets - so all new diesel vehicles are now going to be affected.

It's worth remembering, however, that these costs are included as the car's on-the-road price and are designed to penalise cars that are the highest polluting. As with the rest of the rules, this only applies to diesel cars and not vans.

The amount of tax for older cars remains the same and is based on an emissions-related table introduced in 2001. Any car registered before March 1, 2001 qualifies for a different system of tax, and this one is refreshingly simple. If your car has an engine that's above 1,549cc it's £245 a year, whereas if it's under 1,549cc it'll cost £150 a year to tax.