This is the third Volvo we've reviewed with the new D4 two-litre diesel engine, one of what will become a family of no fewer than eight units suitable for very different applications.I'm looking forward to trying the other seven, because the D4 is, at first sight anyway, a remarkable piece of work. As fitted to the S60, it produces up to 179bhp, yet it performs so well on the EU fuel economy test and has such an impressive CO2 rating that even if you specify the eight-speed Geartronic automatic transmission it will cost you less in annual Vehicle Excise Duty than you probably have in your pockets right now. And if you choose the manual version instead it won't cost you anything at all.Geartronic adds £1550 to the price of the car, but if that's not a problem there is a case for choosing it, on top of the usual one that you don't have to change gear yourself.To explain the supplementary reason I must first refer you to an earlier review of the V60 D4 manual, which had difficulty coping with the same 179bhp when it was applied suddenly, for example when an overtaking opportunity arose. This didn't happen with the S60, presumably because the slower response of the Geartronic box gave the rest of the car time to prepare. So that's a blessing.This doesn't mean that the S60 is good to drive. Hell, no. It chitters over short, sudden bumps and flollops over longer, more langorous ones, and generally feels much larger and heavier than it actually is.I was discussing this very matter (at a location in Mayfair which the membership rules forbid me to reveal) with a colleague who had driven the same car, and he expressed astonishment that a senior Volvo employee could have signed off the car in this state. And it's not as if we had been unlucky enough to pick a rotten apple from the barrel. Another colleague, who had recently driven another S60 D4 in R-Design form, with larger wheels and lower-profile tyres, said it was even worse.To add to the unpleasantness, the back of the driver's seat is very firm and the side bolsters are very far apart - I am not a small man, but I would have to gain about five stone before I could touch both of them at the same time. As a result, every time the car turned a corner my torso pivoted around my spine, and the effort of pivoting it to the original position caused muscular twinges within seconds.Well, at least there's still the fuel economy - or, at least, there's still the low annual VED payment. The official combined fuel consumption figure is 67.3mpg, but I couldn't get beyond the mid 40s in normal motoring. And this wasn't because I was pushing hard. For the reasons given in the foregoing paragraphs, I wasn't going to drive the damn thing any faster than I had to.Normally I would start talking about things like equipment levels at this point, but you know what? I'm not going to. This is the worst Volvo I've driven in years, and the worst current-generation S60 ever, and I'm not going to recommend it to anyone unless something is done to make it less uncomfortable. Engine 1969cc, 4 cylinders Power 179bhp Transmission 8-speed automatic Fuel/CO2 67.3mpg / 109g/km Acceleration 0-62mph: 7.4 seconds Top speed 142 mph Price £29,395 Details correct at publication date