BMW offers drivers a no-brainer choice with the new 6 Series Gran Coupe. The 640d diesel has almost as much power as the 640i petrol. It is as fast to 62mph. It reaches the same top speed. As a six-cylinder turbo, it offers similar sound effects and refinement as admirable as the petrol. Crucially, it has a whole mountainside more torque, making it much more energetic and agreeable in everyday use. It costs less than £2000 more, which is a small price to pay for by far the preferable engine.
So why go for petrol? That’s what I thought when I drove it last week. The petrol is nice, and is fast when you work it hard, something it’s perfectly happy to do. But working something so hard doesn’t quite seem right in a car as elegant as this. This is an engine that works best in a 3 Series, rather than a 6.
And boy, does it work in a 3. The 335i is as charismatic as the best 3 Series of yore, with oodles of power and the most delightful straight six hum. In a world dominated by diesel 3 Series, it’s nice to drive something that feels so authentic.

But what about economy? Well, the key reason for choosing diesel in that 6 Series Gran Coupe isn’t just power or engine delivery – it’s a 13.8mpg advantage and CO2 emissions of 148g/km rather than 181g/km. The case for the 640d rather than the 640i is, on paper, absolute – even more significant than the in-use appeal. And a similar advantage is held by diesel 3 Series over the so-fruity 335i.
However, in practice, things are not quite so clear-cut – and here is where things start to get muddled. When I drove the 335i with relative sensibility, I averaged 47mpg. Honestly, I did. With a little more sense still, I topped 49mpg. Big engine gently used is a fairly lightweight car: just the thing for stretching fuel economy. The on-paper stats may not fully reveal it but even the least-green 3 Series has the potential to be green.
You sense as much from driving it. There’s a real effortless to it, an impression the engine has barely to be worked. A feeling you don’t get in the 280kg heavier 6 Series. In that car, the same feeling of effortlessness is only achieved by something with a lot more torque and a lot more muscle, to use its built-in heft to make light work of everyday use. The official stats show the promise if you’re steady: the reality of driving the 640d is likely to get closer to this than the comparable 640i.
This effect is something seen often in small, tiny-engined city cars and superminis. The stats say they should be averaging one thing, but the real world figures reveal something far removed. That’s because the real world isn’t like the lab, and an engine optimised to perform well here may not necessarily have anything in reserve to do the same in use, too.
Such is the importance of using some investigative skills when looking at the official cycle figures. If something is too good to be true, look deeper: it probably is. A small petrol-engined car with diesel-matching fuel economy on the official cycle actually have a high kerbweight, or blunt aerodynamics, or a simple plain lack of torque that means you’ll never be able to drive it in the fuel-optimised way? Them you’re unlikely to achieve what it says on the tin. Simple physics.
The BMW 6 Series Gran Coupe is clearly a no-brainer. This isn’t always the case though. Only from evidence in the field can accurate judgments be drawn. And don’t think it’s simply down to what engine a car uses, either. I’ve driven VW Group 2.0 TDI engines that have varied by double-digit economy figures simply because of the model they’re installed in – and we’re not comparing SUVs with Golf hatchbacks here. Even apparently similar cars can still vary by more than you’d think.
Fuel economy is a science, but it’s a complicated one. There are ways of simplifying it, which is what the official test aims to do – give car manufactures a strict, defined set of conditions that they can accurately replicate so direct comparisons can be closely drawn. Thing is, as they know the conditions, they can optimise their engines to perform well in them. That’s what leads to the difference between theoretical and actual fuel consumption – some perform better away from the ideal than others.
Those expecting a BMW 640i to be as impressive as a BMW 335i in real-world driving will be sadly disappointed. They’ll need to follow their nose and stick with the car they think will do best. As for the BMW 335d, well, the firm hasn’t even hinted at such a car yet. But, with all that torque and masses of power, I’d say it’s a potential fuel economy hero – on paper AND in the real world. If you can resist using all its searing pace, that is…
Richard Aucock