This shouldn't come as the greatest surprise to anyone ever but sadly it does to those who would plan our lives. If you set people a target for something then they will game that target. This is true whether it's of bankers vying for bonuses and pushing losses outside the bonus period, people making tractors where if you measure them by the tonnage they'll make heavy tractors and, in today's example, if you set car manufacturers mpg targets for their vehicles then they'll game the measurement of mpg:

Motorists are usually advised that smaller cars can travel more miles per gallon (mpg) than those with larger engines, making them cheaper and more environmentally friendly to run.

But manufacturers’ estimates of fuel economy, based on official laboratory tests, may not reflect the reality when the vehicles are driven on the road.

Tests on 500 vehicles, half petrol, half diesel, each driven for three hours on roads in Britain, found that the cars travelled on average 18 per cent fewer miles per gallon than stated in manufacturers’ specifications.

Emissions Analytics, a data company which measured the cars’ fuel consumption and emissions, explained that this was due to cars accelerating more and travelling at higher speeds on the road than in official testing regimes.

The test drivers are of course all practicing perfect economical driving while, and this problem is worse for very small engines, we real drivers give it a bit of welly to compensate for those very small engines. And of course this is a general fault with any and every target of this type. Showing, once again, that if you want to ration something (here the desire is to ration fuel consumption) the best way to do your rationing is by price. For price is the one thing that doesn't lie in these circumstances.

It's such a simple concept that it really is amazing that those who would plan our lives still don't get it.