Can anyone shed light on this problem? Chrysler Grand Voyager 2.8 Diesel.

My trip computer tells me I am getting 30 MPG, in actuality I am only getting 20 MPG. This error is consistent when the computer tells me 40 MPG I only have 22MPG.
A motorway run returned less than 26 MPG.
I am not asking about the MPG, just the trip wrong computer reading.

The main dealer in Chester has no idea what it means.
How can the computer be so wrong?
It cant be an engine fault causing the abysmal mileage. because the computer would know I was getting low MPG and give that readout.
I drive with a very light foot.

 

It seems to be a common problem with Voyagers. There's been lots of posts which include mention of this.