I bought one of these recently, a Honda Accord Diesel.
People who've bought them have complained that they don't achieve
Honda's quoted consumption. My experience initially in mine was
similar. I was getting about 42 mpg in mixed driving (British gallons
here).
I've had the car about 2 months.

Then I started experimenting. The car is so powerful with such
acceleration that one is inclined to sink the welly when driving.
I've found that if you use the highest gear you can, and accelerate
gently except only when required, that you get much better
consumption. You say "what's new". It's the degree is what amazes me.

Last night I drove about 33 miles to a small town outside Cork City
where I live. Country driving. Driving more or less normally, but no
hard driving. 55mpg.
On the way back I used 6th gear a lot and really went for it. My eyes
popped. 4.2L/100km = 66 mpg. I've never driven a car that could do
that. My previous car, the excellent Peugeot 406 HDI diesel maxed at
60mpg.