I do feel slightly sorry for the car manufacturers. There are so, so many variables that affect car fuel efficiency that it is hopeless to ask for a prediction that can be both accurate and precise. You'll have to leave it so wide that it's largely meaningless (eg 20-40 mpg), or you'll be wrong more often than you're right. You don't know how the car will be driven, who'll be driving it, how fast, in what weather, carrying what passengers and luggage, etc etc.
That said - there is no excuse for having a test that allows them to test the car driving in a totally unrealistic way, with equipment removed, vents taped over, engine blueprinted, ludicrous tyre pressures and hybrid batteries precharged. It's just fantasy, and helps noone. Not even the marketing people, as every now knows it's totally rubbish to suggest your 2 tonne 150mph German executive car will average 60mpg.
Personally. I'd like to see them required to fit every car with an *accurate* economy gauge, with some kind of wireless link attached. Everytime you fill a car up it reports it's economy since last filled, chassis type and engine type (eg 2016 Ford Focus Hatch, 125bhp petrol Ecoboost). Drivers can then go to a government website to see what their proposed car will manage in the real world.