Re: Only 10%?
"The intention is that it's used purely as a relative comparator, not an absolute, so that you know that a car with an official consumption figure of 70 mpg will be more efficient than one with a 60 mpg figure, and not that either will actually deliver the quoted consumption figure"
But it's still a useless comparison. You have a car that the MPG is lied about compared to a car that has MPG that you'll never get. So you may as well spit up in to the air and see where it lands because you simply can't make any sort of decision on it.
Official MPG is a useless yardstick at the moment, regardless of lies or truth.