I don't know if 84mpg in Passat is true or not but I know that newer cars have some gizmos build in and you don't have to think about killing the engine because it has start/stop function. Modern cars have cylinder deactivation (new VWs for sure), and modern engines are more efficient than older ones.
For example my wife's Mietek (MB w168 cdi) has 1.7 liter direct injection 16 valves diesel engine with 90HP like my Berta's engine. All the parameters are very similar, Berta have even slightly more torque, but it's an indirect injection diesel. And I have to fight hard to have 3.7l/100km (63.57mpg US) in Berta and it was fairly easy in Mietek with A/C turned on from time to time. On the same route. And Mietek is less areo than Berta, and it's automatic. But it's a next generation of diesel engine.
As you can see modern cars are more "eco" than older ones. We have lots of commercials recently of new cars and they say that for example average fuel consumption of new Citroen with 200HP engine is 3.4 l/100km (69.18 mpg US), what is hard to believe. But maybe? I would like to take that car for a spin and see on my own
