Hi Ya, really do need more details. BUT, sounds like two issues. start off by doing the almost free and most likely causes, which are many. i would automatically start with your MAF ( mass air flow sensor ). The slow deceleration could well be a sticking throttle body as your cars second issue. sort these two things out, disconnect battery for five minutes and the connect and run for a few miles. your car will need to reconfigure and will give you low gas milage, the car needs a 20 min drive usually... after it has sorted itself out you will notice a difference. good luck !

About company fuel efficiency numbers
a few car companies world wide have been caught publishing misleading fuel consumption figures
all sorts of cheats

i would say your diesel is running normally

if you keep taking back to the service division saying poor fuel efficiency
they will take the car for half a day
it will cost you the most .. not Izuzu

the sales pitch
Combined Fuel Consumption#
MU-X 4X2 LS-M
AUTOMATIC
8.1L/100km

No, its your problem reading them (STOP) and WIN.
try to learn the economy is totally up to the drive.
I can get huge economy but son cant,
he has heavy right foot,
it like predicting the weather, mostly silly. unless NOW.
it varies by.
speeds, hills, driver, tire inflation. altitude, dragging breaks, slipping transmission, age of engine, or if never tuned up, and the driver.
and more, even weather,! and grade of fuel,

why not join a MPG forum that has 1000s of others with same car and they all report mPG, (or L per 100km) (city page, country page or open road MPG)
and learn what is real.
All most all cars new, that number is computer generated,
that is a fact! so who do you believe a silly computer model or real owners.

good luck doing all the bs , the guessing people do here
they will have you broke fast, I promise.
but if you ask the blind what do you expect, $$$$$$$$$$$$$