Diesel cars 'better than hybrids' for fuel efficiency

Some diesel cars are giving the much-hyped hybrids a run for their money when it comes to fuel efficiency.

Toyota's petrol-electric Prius, which kick-started the fashion for hybrids a decade ago, achieved fewer miles to the gallon than a sporty BMW 3-series diesel, according to consumer watchdogs.

Many more diesel cars are running neck and neck and giving the hybrids a run for their money, the Which? research shows.

Electric: The Toyota Prius was compared with the sporty BMW 320ed

And with customers generally paying a premium to buy a hybrid, which is generally a few thousand pounds more expensive than a standard petrol or diesel model, the extra cost of the car may outweigh the fuel saving.

The news comes despite the price of diesel yesterday hitting a new record £6.50 a gallon.

The latest edition of Which? Car looked at the actual fuel efficiency achieved in 'real world' driving rather than the claims made by the manufacturers.

This showed that in its tests a BMW 320Ed small executive saloon managed 64.2 miles to the gallon - ahead of the Prius's 61.4mpg. Over a year's driving of 12,000 miles, the BMW will cost £1,120 - or £51 less than the Prius at £1,171.

The Prius and other hybrids rely on one major leap forward in technology.

The BMW relies for its fuel efficiency on what it terms 'efficient dynamics' - that is making lots of small efficiency gains across the length and breadth of the vehicle - from lighter seats or chassis to more frugal engines - which, when put together add up to one major gain.

The Prius is called a 'hybrid' because it combines a conventional 1.8 litre petrol engine with an electric motor which work seamlessly together.

The electric motor operates as a generator to help recover surplus energy, including from decelerating and braking, that would otherwise be wasted. This energy restores the charge of the high-power battery, which in turn can be used to power the car's wheels.

Diesel power: In the off-roader category, a Lexus RX 450h hybrid was £78 cheaper than a diesel Volkswagen Touareg

To cut fuel consumption and reduce harmful emissions, the car's electric motor takes the vehicle from rest, and lets the engine cut in when it is in motion. The two engines then dove-tail to maximise fuel efficiency and minimise emission levels. As a result, emissions of carbon-dioxide (CO2), the so-called 'greenhouse gas' blamed by scientists for global warming, are slashed. And fuel economy is boosted massively.

But Which? Car notes:'In most cases you'll save money with a hybrid. Fuel costs are low and they save drivers on tax, not to mention lower emissions.

'However, they're not always more fuel-efficient than advanced diesel engines. Which? Cart has found that a diesel BMW 320Ed (fuel costs for 1 year or 12,000 miles: £1,120) would cost you less at the pumps than a Toyota Prius (£1,171).'

Of the BMW, the report noted:'BMW may have a reputation for making aggressive, sporty saloons, but the 320Ed (Efficient Dynamics) makes an impressive statement about BMW's clever fuel-saving technology.'

It noted:'At first glance it's a standard 320d saloon, but the detail improvements lift claimed average fuel economy from 60.1 to 68.9mpg. We didn't quite match this, but the BMW did beat the Prius in our mpg tests.'

However, the report notes that hybrids do have some advantages:'It's important to look beyond just the headline economy stats.

'The difference in pump prices - with diesel around 6p a litre more than petrol - and the potential tax liabilities of each car, could also sway your decision.'

The Prius has CO2 emissions of 89g/km which mean it is free of car tax, compared to just £20 for the 109g/km BMW (though zero tax in the first year). Which? notes:'The Prius's lower CO2 emissions could also save company car drivers nearly £400 a year over the BMW.'

Among other models, the hybrid Honda Insight SE has an annual fuel bill of £1,171 - just £25 ahead of the diesel Skoda Octavia Greenline II at £1,196.

Among off-roaders, the Lexus RX 450h hybrid costs £2,115 a year for fuel, £78 cheaper than the diesel Volkswagen Touareg 3,0 litre TDi at £2,192.

Toyota' Auris Hybrid T4 costs £1,120 a year to fuel - £129 less than the diesel Vauxhall Astra 1.3 CDTi SRi.

The cost of diesel has reached the equivalent of £6.50 a gallon, according to AA figures. On average, motorists are paying 142.99p a litre (or £6.50 a gallon) for diesel, with petrol now at 137.24p a litre (or £6.24 a gallon).

A year ago, average petrol prices were 121.43p a litre, with diesel at 122.83p.

Compared with a year ago, it is now costing nearly £8 more to fill a typical 50-litre tank with petrol, while filling up with diesel is now more than £10 more expensive, the AA said.

It added that a two-car family was now paying £33.57 more a month for petrol than a year ago, while the extra cost of filling an 80-litre diesel tank in a commercial van was £16.13.