You have to be really careful when talking about fuel economy. The numbers mean one very specific thing, on a particular test the car achieved a particular fuel economy. Without specifying how exactly they got those fuel economy numbers(referring to article), the numbers are literally meaningless. If I'm just cruising along at 60mph on flat land I can get 65mpg in my gen2 prius, even though the official highway number is 45mpg.
Driver behavior, AC, the test conditions, and vehicle conditions all affect efficiency.
With the proper driving strategy the prius can great efficiency out of situations that are problematic for traditional vehicles. What kind of fuel economy do you get out of your diesel in stop-and-go traffic? With regenerative braking and the ability to turn off the engine when not needed can easily get 75mpg+. On the freeway with the cruise set at 70mph (cruise control is not optimal) I get about 53MPG.
The 2009 Prius I drive has an official US combined MPG of 46. On average I get 55 MPG. The regenerative braking has great potential, if you drive in a way that takes advantage of it by planning your stops.
If you look at the numbers:
40 MPG is 5.88 l/100km 55 MPG is 4.28 l/100km
(5.88 - 4.28)/5.88 = 27%
The prius is 27% with my numbers and that's against a fuel that is 11% more energy dense.