Because most drivers cannot replicate the the testing sequence, and a majority of drivers can pass a driving test but have no idea how to acturally, rather than just stick it in drive and push the stop & go peddle.
Don't think those figures are there for someone to attempt to replicate them. It is more of a guide
line. If the testing environment is controlled and able to be replicated, all manufacturers are able
to produce those figures as a guides. The equivalent of lab testing or level playing field.
As a consumer it is easy to compare one vehicle with another but not necessarily be able to
achieve those figures.
Although as a consumer you may not be able to replicate them, the idea is to demonstrate
one vehicle is more/less efficient then another.
Not to forget that any car doing fuel benchmark will have no mud flaps, roof rack, aerials, footwell mats or any other accessories to save weight & improve aerodynamics.
It will be the "pauper" model & not the "keeping up with the Joneses" model. Which doesn't have electric motors powering everything, like 24 way massage seats with heating & air conditioning, or 2000 watt 24 speaker hi-fi system.
Footwell lighting, puddle lights, mood lighting, star ceiling, fridge all add weight to the Joneses model.
It will have the narrowest & hardest tyres approved for the model.
A brand new car will be a bit lighter as it is not carrying any dirt (an old 747 is 2 tonnes heavier due to dirt).
Doesn't matter how hard one tries you simply will never match a benchmark other than with a tailwind.
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