The first three cars in the DRIVe line, the C30, S40, and V50, were all designed to register below 120g/km CO2, and in fact all slipped in at 107g/km or lower - all about the same as a typical Toyota Prius. The four new additions add two more tiers of efficiency, with 140g/km and 160g/km as their goals.
This allows room for larger cars while still retaining a legitimately green (and cost-saving) image. The V70 and S80 both rate at 129g/km, while the XC60 and XC70 rate 159g/km, just squeaking in below their 160g/km target. To help get these low figures, the V70 and S80 get 1.6L diesels, while the XC60 and XC70 are offered with FWD powertrains.
Those emissions figures translate into very respectable fuel economy as well: the V70 and S80 DRIVe models get 48mpg US (4.9L/100km) while the XC60 and XC70 rate 39.2mpg US (6.0L/100km).
Like their DRIVe predecessors, the new models get these high figures through a combination of a wide range of technologies. Some of the trick and techniques include taller gear ratios, start-stop function (only on the C30, S40 and V50), regenerative battery charging, and improved aerodynamics. Mercedes and BMW have done similar things with their BlueEfficiency and EfficientDynamics lines, while GM offers its XFE line with many of the same modifications.