You expect to pay extra for a little slice of luxury but, when it comes to buying a luxury diesel car, don’t assume that the fantastic mpg quoted by your dealership is guaranteed to be another one of those extravagant extras.
Unique Which? testing has revealed that the advertised mpg shouldn’t always be taken at face value. For example, the BMW 6 Series Gran Coupe is 15% less efficient than the official figures claim. That’s an annual difference in fuel costs of £208 per year based on driving 10,000 miles per year at current fuel prices.
At the other end of the spectrum, the Audi A8 and Audi A5 Sportback both fall within 5% of the advertised mpg. Each car will cost only around £40 per year more based on our cost calculation, so won’t leave you with such a large unexpected outlay at the fuel pump.
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Why the mpg gap?
EU law requires carmakers to show official test figures in their adverts to help consumers compare fuel economy between different models. But we think the figures should also reflect real-world driving – and if a brand quotes an mpg figure in its adverts, a consumer should be able to achieve it in normal use.
That’s why it’s vital the EU test is updated to be more accurate in 2017 as planned, and not delayed until 2020 as some manufacturers would prefer.
Does your car deliver the fuel economy you expect? Scroll down for our list of the most misleading models and the extra costs owners could incur.
Luxury diesels – difference in fuel costs per year
In 2013/14 we tested 200 cars in our lab, putting all models through the same test cycle. Here we list the luxury diesels we tested so you can see how far they stray from the advertised mpg.
Annual fuel costs are based on driving 10,000 miles a year and the average cost of fuel for the months March 2014 to February 2015 according to the AA fuel price report (petrol: 125.5p per litre; diesel: 130.0p per litre).
How we test mpg
The testing that underpins every Which? car review is designed to give consumers the most accurate picture possible of how their car will perform in everyday life. So when assessing fuel economy, unlike the official EU test, we include a motorway driving simulation, switch on the lights and air-con, and inflate tyres to the recommended pressures.
And if a car has different driving modes available, we use the one it starts up in by default, rather than switching to an ‘eco’ mode. This may offer better economy, but will also often neuter a car’s performance to the point where it’s awful to drive.
What’s the problem with EU mpg measurements?
Don’t be fooled by the name of the official driving simulation currently used to calculate mpg figures – the New European Driving Cycle (NEDC) hasn’t been new since it was last updated in 1997, and the basis of the test was first introduced in the 1970s.
The test’s lack of real-world driving scenarios and numerous loopholes makes the figures it generates unachievable when you get behind the wheel of a car.