Smart under fire over fuel economy claims

Recently the BBC program Watchdog investigated Smart regarding the new fortwo Diesel fuel economy claims of 85.6 mpg after a number of drivers had been sent complaints to the program. Smart car owners have found that the claim of 85.6 mpg is almost never seen and in reality they average 50-65 mpg.
So where did the claim of 85.6 mpg come from?

The Smart fortwo Diesel has an emission certification of 85.6 mpg for both EC Urban and EC Extra Urban.
EC Urban (ECE1) and EC Extra Urban (EUDC) are both standardized tests performed on cars sold in the EU. Cars are placed on a rolling road and every car is driven the same way for the same distance and time. The amount of fuel used during these tests gives the EC Urban and Extra Urban ratings, more information here.

So why do Smart owners almost never see the fuel economy found in the test?
Well quite simply because no one will ever drive on the road as it was driven in the test conditions, indeed it is likely that the car itself may have been deliberately designed to perform well in the test. The EC economy ratings are really only good for comparing one car against another and should not be taken as an estimate of how the car will perform on the road.
You can never expect to drive on the road in the same conditions and in the same way the car was driven under the test. Your cars fuel economy is affected by tire pressures, weight, weather conditions, driving style, the age or the car, its temperature, the type of fuel used, ...
However, overall the EC Urban and Extra Urban are often a good way to compare fuel economy. Even if the Smart fortwo Diesel was designed to excel in the test it is still likely to give better fuel economy than a 2l Diesel Peugeot 307 which has an EC Urban and Extra Urban rating of 36.7 mpg & 55.4 mpg.