Just to rub salt into the wounds, BMW is also now offering it as an 'M Sport', the big-wheeled, M GmbH-bodykitted trim so loved by UK buyers that comes to the 6 for the first time (for ?4,665 more than base SE). So yes, it now even looks the part, and no - although the ride is more pattery as a result, it does not descend into resolute harshness.
Let's amuse ourselves by comparing 635d with 640d. Power of the new 3.0-litre straight-six? 313hp versus 286hp, a genteel 27hp boost. Not bad: torque is also up 37lb ft to a healthy 464lb ft, spread in the modern diesel way across a 1000rpm band starting from just 1500rpm. That's TwinPower Turbo tech at work: two different-sized turbos operating at different points to boost response and effectiveness throughout the rev range.
Economy is the clincher. The old one could do 40.9mpg: this one averages 51.4mpg. That's 10.5mpg more, despite all the extra pace and shove. It's barely comprehensible: nor is a CO2 drop of 39g/km, to just 144g/km.
Grand Tourer? You bet: with a 70-litre tank, this has a 790-mile range, 225 miles more than the 640i and 361 miles more than a Mercedes SL 500.
Once in its stride though, it's not only more serene but staggeringly smooth and remarkably free-revving with it. The aural backdrop is, intriguingly, not dissimilar to the distant yet cammy growl of a water-cooled 911. There's more to appeal at 4500rpm than you'd ever expect.
And boy, is it a powerhouse. The torque-laden surge on tap apparently at all revs provides an amusing old-school muscle car feel. Even with DSC+ on, you can feel the strain on the rear wheels, made all the more convincing by the fact you don't have to wait for it to be generated. Turbo lag is virtually never an issue: power delivery is near-instantaneous especially when aided by a 'Sport' mode which means you're pretty much always in the right gear.
Otherwise, all that's missing is a bit more insight from the otherwise accurate and firmly weighted electric power steering. It's as clean and crisp as most other new BMWs, but you don't get a huge amount of detail. And what's with the over-large, over-thick, over-spongy steering wheel?
It's a fleeting chink for the competition, but relatively inconsequential. Where BMW has not faltered is over the fundamentals. Fast, effortless and satisfying, the 640d hits the spot like no modern 6 Series before it. A force-filled grand tourer that packs in the attributes of a diesel with few of the downsides, and possibly one of the most well-rounded BMWs on sale.
Words: Richard Aucock