Once upon a time, choosing the engine for your car was simple. It was petrol or nothing. If you wanted diesel, you chose a lorry – or a London taxi.

Up to the late 1970s, diesel engines were too noisy, smoky and slow to be a realistic choice for a family car. But then came car diesel engines that were quieter, more powerful and civilised. Now diesel has become a real choice for everyday motoring. More recently still, the choices have widened with the appearance of electric cars and hybrid vehicles like the ground-breaking Toyota Prius that use smaller petrol engines with electric motors for additional power. But which of these actually delivers lowest running costs?

Petrol versus Diesel

Diesel versus petrol calculations hinge on running costs. The petrol option will always be cheaper to buy with today’s fuel-efficient designs unless you are covering more than 15,000 miles a year and keeping the car for a long time. This is because diesel models cost more to buy than their petrol-engined equivalents and also because diesel fuel is more expensive than unleaded petrol in the UK.

The key diesel advantages are that they tend to use less fuel and crucially, emit lower CO2, making them cheaper to tax. Diesels also tend to cost less to insure and retain higher residual values at trade in time. This is because most buyers perceive diesels as more reliable, more economical and longer lasting. The whole life running cost advantage for diesels is even more pronounced for company cars, where Benefit In Kind (BIK) is calculated using list price and CO2 emissions, with tax paid on the result. Car-leasing companies that work out whole-life costs taking into account purchase price, emissions, economy, depreciation, maintenance and employee National Insurance details normally find the results favour diesel cars, by up to £30 a month.

Servicing and reliability are more or less the same for diesels as petrol engined vehicles. However, there is a big difference when it comes to making a mistake at the fuel pump. Putting petrol in a diesel car will normally not immobilize it. The damage is even worse when a petrol-engine car is mistakenly fuelled with diesel, requiring companies like GEM Motoring Assist to get you home. Thankfully, modern fuel filler and pump nozzle design, where diesel nozzles are too large to fit modern petrol cars, make such a disaster less likely.

Fossil fuel versus green

When it comes to electric and hybrid cars, the calculations are more complicated still. Yes, electricity is very cheap compared to refined fossil fuels, particularly when obtained from a mains socket, but the high amperage power supplies designed for vehicles are few and far between. With a purely electric car it is too easy to run the batteries completely flat if venturing far from home and be left stranded far from the nearest recharge point. Hybrids avoid this problem but are much more expensive to buy (even with the Government rebate) and also more expensive to maintain, simply because there are a lot more components in the drivetrain, including some very expensive electrical systems that are only available from the manufacturer. You will also be tied to the official dealer network for all maintenance, since few local garages have the specialised equipment needed to service these complicated vehicles. The main problem for electric and hybrid vehicles though lies in the batteries. The fearfully expensive lithium units do wear out after a few hundred charge/discharge cycles. Replacing a vehicle battery pack can cost many thousands of pounds.

So which is the best engine for you? For more energy efficient, environmentally friendly, we think diesel is the way forward.