Drivers warned of the least efficient petrol and diesel cars on the market - Ford, BMW, Audi, VW and more

New research has found that the Audi S8 saloon, which costs more than £100,000, has an average shortfall of 11.78 per cent between its What Car? "Real MPG" figure and data from the official Worldwide Harmonised Light Vehicle Test Procedure (WLTP) figures.

In official WLTP testing, the S8 returned a fuel economy rating of 24.6mpg, while independent testing found that its actual on-road performance dropped to 21.7mpg.

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Of the top 10 worst-performing vehicles in the Real MPG table, only the Ford Ranger Raptor 3.0 Ecoboost 292 4WD officially underrated the performance of the vehicle.

In official tests by the company, the large pick-up truck was found to produce just 20.5mpg, whereas the What Car? Real MPG test found a 17 per cent improvement to 24mpg, although it still saw the vehicle have the second-worst record.

The testing was conducted under "strictly controlled laboratory conditions" based on driving routes that replicate rural, urban and motorway driving, by also sticking to speed limits without resorting to slow acceleration and hypermiling techniques.

Popular vehicle brands including Audi, Ford, BMW and Volkswagen also appeared on the worst Real MPG performers list.

The Ford Focus ST 2.3 Ecoboost 280 was found to have the biggest difference between miles per gallon ratings. In WLTP tests, it was found to have 35.5mpg, although What Car? testing found this would drop to 29.58mpg - a difference of 16.67 per cent.

Steve Huntingford, What Car? editor, said: "The broad spectrum of models included in our list of the best and worst performers shows that efficiency ratings aren’t necessarily dependent on a vehicle’s size, category or even price.

"And two of the vehicles in our top 10 even exceeded their official figures, which shows the importance of thoroughly researching your next new car."

The average Real MPG figure for the 10 least efficient models came in at 28.3mpg, compared with 54.4mpg for the 10 most efficient cars in the standings.

Cars ranked by What Car? Real MPG values

1. Audi S8 - 21.7mpg
2. Ford Ranger Raptor 3.0 Ecoboost 292 4WD - 24.0mpg
3. Cupra Ateca 2.0 TSI 300 - 28.5mpg
4. KGM Musso 2.2 D - 29.0mpg
5. Audi Q3 45 TFSI - 29.5mpg
6. Ford Focus ST 2.3 Ecoboost 280 - 29.58mpg
7. BMW 4 Series Convertible M440i xDrive - 29.59mpg
8. Audi Q5 Sportback 45 TFSI - 29.8mpg
9. Volkswagen Multivan 2.0 TSI - 30.4mpg
10. Alfa Romeo Stelvio 2.0 Turbo 280 - 30.9mpg

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At the opposite end of the spectrum, the Toyota Yaris Cross 1.5 Hybrid and Toyota Yaris 1.5 Hybrid 116 were found to be the most efficient vehicles, although the Real MPG rating saw a four per cent and 13 per cent drop respectively.

Other popular brands and models like the Seat Ibiza, Toyota Aygo, Audi A4, Honda Jazz and Kia Picanto were found to have some of the best miles per gallon rates on the UK market.