Real MPG: the least efficient cars revealed

What Car?'s laboratory tests use a real-world driving route to show what fuel economy you can really expect. But what are the worst performing cars we've ever tested?...

True MPG testing

So, you've seen the most efficient cars, all of which average more than 55mpg in real-world driving conditions, but which cars use the most fuel?

Here we answer that question and see just what they do deliver if you’re driving gently and sticking to speed limits, but aren’t resorting to any unrealistically slow acceleration or special 'hypermiling' techniques.

1. The least fuel-efficient car - Audi S8

Audi S8 2020

Real MPG 21.7mpg

It offers savage straight-line performance and is incredibly comfortable, but there's no getting around the fact that this go-faster version of the Audi A8 luxury limousine will be hideously expensive to run. Still, if you can afford to put one on your driveway in the first place, running costs aren't likely to factor among your biggest concerns.

Read our full Audi S8 review

And the rest of the bottom 10...

2. Alfa Romeo Giulia Quadrifoglio

Alfa Romeo Giulia Quadrifoglio

Real MPG 23.5mpg

Like so many cars in this bottom 10, the Giulia Quadrifoglio is designed for performance rather than efficiency. Indeed, at the 2023 What Car? Car of the Year Awards, we named it the best performance car for thrills, which tells you just how good it is at the things it specialises in.

Read our full Alfa Romeo Giulia Quadrifoglio review

3. Lexus LC500 Cabriolet

Lexus LC Convertible front cornering

Real MPG 24.2mpg

With this open-top version of the Lexus LC, you can look forward to breathtaking performance and turning the heads of other road users as you pass. However, you can also 'look forward to' visiting the petrol station often, courtesy of the 5.0-litre V8 engine.

Read our full Lexus LC review

4. Audi SQ5

New Audi SQ5 vs Mercedes-AMG GLC 43

Real MPG 24.6mpg

Like a lot of sports SUVs, the SQ5 is as heavy on fuel as it is light on its feet – or at least it was until Audi swapped the petrol engine for a diesel. Strengths also include a beautifully built interior.

Read our full Audi SQ5 review

5. Mercedes S500 Cabriolet

Mercedes S-Class Cabriolet

Real MPG 25.2mpg

Until it was discontinued in 2020, the S-Class Cabriolet was the best luxury drop-top you could buy. However, it was only for those with a big balance, due to its high price and running costs.

Read our full used Mercedes S-Class Cabriolet buying guide

=6. Mercedes-AMG GLC 43

Mercedes-AMG GLC 43 front

Real MPG 25.3mpg

If you get your thrills from blasting away from traffic lights and listening to a piercing V6 soundtrack, the GLC 43 SUV is well worth a look. It’s even practical and easy to drive when you’re in family mode. But it's never what you'd call frugal.

Read our full Mercedes GLC review

=6. Porsche Macan Turbo Performance Pack

New Mercedes-AMG GLC 63 vs Porsche Macan

Real MPG 25.3mpg

The Macan Turbo is a rival for the GLC 43, and averaged the same 25.3mpg when we put it through our Real MPG test. However, it's since been given a new engine and is the more rewarding car to drive.

Read our full Porsche Macan review

8. Land Rover Discovery 3.0 SDV6

Land Rover Discovery 2021 front

Real MPG 26.3mpg

Large, luxurious SUVs aren't an efficient breed, but even by the standards of the class the Land Rover Discovery gulps down a lot of fuel, barely topping 26mpg.

Read our full Land Rover Discovery review

9. Mercedes S500

New BMW 7 Series vs Mercedes S-Class

Real MPG 26.9mpg

The S-Class has long been one of the best luxury saloons around and is extremely relaxing to drive or be driven in. However, despite featuring mild hybrid technology, the previous-generation S500 version is anything but frugal. A new version has since gone on sale.

Read our full used Mercedes S-Class buying guide

10. Volvo XC40 T4

Volvo XC40 D4 R-Design front three quarters

Real MPG 27.1mpg

We're big fans of this family SUV; it was even our 2018 Car of the Year. However, while this turbocharged 2.0-litre petrol version is nice to drive, a Real MPG score of 27.1 makes it significantly harder to recommend.

Read our full Volvo XC40 review

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