I would say it depends on the driving-I "hypermile" and used to get 78mpg out of my diesel Panda-over the official mpg, 60mpg from my Sirion 1.0S and 58mpg out of the Perodua Kelisa.
A friend runs a power-chipped Skoda Superb 1.9TDI and averages 72mpg on his daily 50mph B road commute-we have checked and checked this against actual fuel, not the computer.
My routes are 50mph constant, no traffic so I usually beat the average, but always get somewhere between Urban and Extra Urban-the Greenline Skoda is 55.4 and 67.3....so 55 is poss in my area.
It's difficult to hypermile in the Omega-its auto and very very heavy, hence my average 28mpg over the month 
I find that quite worrying really... I get 26.8-28.3 indicated on the MID (which I've confirmed accurate) as what would be my petrol consumption, and that's in a 3L, with lots of roundabouts and not exactly hanging about

Getting it looked at again tomorrow-cleaning throttle bodies again etc. It's had every possible fluid changed, complete exhaust, cat, manifold, gasket, new brakes/discs/fluid, tyres+ pressures, software updates, HUGE service, tech2 checks and 500 other jobs-it should be at peak efficiency...
Many thanks to welung666, today we did the following to try an get the car finished once and for all:
"My Naff Code Reader" session
New rocker cover gasket (again)
DIS pack checked, DIS boots shot-need to get
Selector lock and brake pedal switch sorted
Rear suspension/bushes checked
Front wishbones checked/torqued
Power steering pump/pipes checked, clips tightened
Drivers door stay welded, lubricated
Drivers door handle removed, lubricated, refitted
Bootlid adjusted to cure rattles
Speakers checked for rattles-to replace NSR/NSF
Steering column shroud rattle sorted
All towbar Scotchlocks removed and new connectors soldered
Towbar socket replaced to cure beeper fault
Towbar electrics checked with light bar
Exhaust clamps tightened
and more, lots more....
If it wasn't for OOF and people like welung666 I would have gone mad by now.
At least now I can keep it with some confidence now-jus the DIS boots and bootlid to sort now.
Cheers welung

Triggers Broom lives another day.....