LET’S TRY AN experiment: the new BMW 330d on a long, empty stretch of flat road, in sixth gear with a trailing throttle, decelerating from 25mph to the 3.0-litre diesel’s idle speed of about 700rpm.I’m slowing, waiting, expecting it to stall. Only it doesn’t. Instead, the 330d settles to about 18mph and drags along, at idle, in top.Foot hard down: rumble from beneath the bonnet and, very slowly at first, the BMW builds speed. From 1000rpm on there’s meaningful acceleration, and by 1750rpm – where torque enters a 369lb ft plateau and there’s already 120bhp – it’s on the pace.I’d continue the experiment, but by now the 330d’s doing 70mph. Repeated in lower gears, there’s smooth, progressive power from a little over 1000rpm, past the 4000rpm peak of 228bhp (up 24bhp over the previous 330d), to maximum revs at 5000rpm.So the 3.0-litre diesel in the latest 330d is an astonishingly capable engine – and quiet and economical, too. At idle, it has a muted rumble, which at cruising speeds is inaudible, and at high revs not intrusive.Throttle response is almost as linear and quick as the 3.0-litre petrol’s, and while the car’s trip computer read 35mpg the official 43.5mpg is possible on a lengthy motorway run.And the rest of the driving experience? Pure 3 Series. The ride, on 225/45 R17s, is fidgety but acceptable, while the chassis is balanced and exploitable. The six-speed gearbox is smooth, driving controls are weighty and progressive, while minor switchgear is of high quality.Okay, both iDrive and the manual seat adjustment still annoy us, but other than that, this car is superb.