I've found that no car I have ever had has matched manufacturers fuel economy figures.
Reading some articles in the motoring press this is apparently down to the way that the figures are obtained. Manufactures put an engine on a bench in a workshop hooked up to a computer and test them in perfect conditions.

You and I actually drive the cars on a road. There are therefore 101 ways in which we will never match the figures.
Road surface variations, inclines, headwinds, stopping and starting again, load carried, slightly off tyre pressures in the car etc etc etc.

If you look at the manufacturers fuel consumption figures for motor cycles you would believe that the average car these days is more efficicent with its fuel than the average 600cc motorcycle. This is because motorbikes are tested more realistically to obtain the fuel economy figures.

However economically you try to drive your car I suspect you will never match the quoted figures, and I believe the manufactures figures to be a bit misleading. I don't rely on them and it is probably best just to use them as an indicator of how fuel efficient one car can be when compared to another.

Finally how do you rate the Dune in other aspects as I fancy one when I come to change my car