Can you trust fuel figures?

Once we stopped believing in official mpg, we went for a diesel

I do sometimes wonder what official fuel figures are for. They are achieved in such ideal conditions, no one can ever replicate them on the road.
   The argument is that they allow you to compare one car against another, but when my husband and I wanted to buy a car, we were sucked into looking at entirely inappropriate cars, based on their official mileage. See the story here).
   We only got wise when we looked at the trip computers of the cars we were test-driving. It's something I always do now when driving manufacturers' cars, and I suggest you do it, too, if you test drive a new car. Without fail, the economy recorded is always way off the quoted figures. (yes, I know the economy would get better once they'd got a few more miles on them, but not that many!)
  Consumer motoring website, HonestJohn.co.uk, has recently published its Real MPG Report 2015. One of its surprising findings was that new small cars, which are often sold to motorists with the promise of superior fuel economy, were among the worst when it comes to meeting their official MPG figures – some of these cars fell below manufacturer’s stated figures by as much as 32.3%.
   Looking into the data, Honest John found that the worst performer was the Nissan Note - which misses its average fuel economy rating by almost a third (32.3%)  The best performing car was the Land Rover Defender - achieving 106.3% - however this year the Defender is going to be taken out of production.
   On average, HJ reckons cars achieve 86% of their official figures.

See Honest John's Real MPG Report.
What Car? also produces its own True MPG figures