Hi chocolat
Driving at a constant speed on a flat smooth surface and in a straight line with still air and consistent humidity and temperature would no doubt result in consumption figures closer to those quoted by any manufacturer. For example any gradient, so matter how slight would obviously require more fuel to maintain a constant speed, you only have to select cruise control and watch the consumption display to see that.
Don't misunderstand me, I can't achieve any where near the consumption figures quoted by any manufacturer but at least their figures allowed me to make a judgment one with another when it came to choosing a car.
The only car that came close was my BMW, which did around 65mpg on the motorway, against an average combined of 65mpg. SO that was pretty good!
Where I think the test figures are misleading, is the i30 is claimed to do 74mpg on average, yet I am struggling to get mid 50's on the motorway. The differences between claimed mpg amongst the manufacturers and what their cars will then actually do seems to be the inconsistent part which makes the judgement call harder.
If you could say that cars will only be at most within 10% of the figures, then that's fair enough as you will have a comparison to look at, but with the i30 being over 30% out in the real world, you'd have to question the validity of the tests.
