Some of the claims manufacturers make are way wrong. How often do we drive at steady speeds? Motoring is full of junctions, bends, lights, hills, dozy pedestrians and other hazards that mean you are using the gearbox and brakes all the time. No wonder fuel consumption never reaches the figures thrown at us.
The best fuel figures I ever had was in the 70's in three cars an Imp (875cc) and two Skoda's a 1000MB & S100 (988cc).
The reason was they were light (under 800kg) roads were quieter, less traffic lights, no dozy folk with MP3 players and phones, so much more free flowing, I rarely dropped below 50mpg in any of these cars.
To produce the torque and BHP of the TA you still need fuel, and the TA produces more than double the power of the Imp for the same CC its also 300+ kg heavier so no matter if its more efficient and the imp was very efficient it will use more fuel, especially if you use the right foot, so perhaps think its impossible to get something for nothing. To get an average 50mpg you'd need to drive slower with less acceleration than I did in my Imp etc.