Thanks for the quick replies. Im going to leave it for a while. As stated its likely to jump about a bit after recetly being zeroed. But I do know it did go wrong. As it usually requires 150 miles or so for any increment to occur. Unless I was to do only town driving then it will obviously drop rapidly. I'm driving to Spain soon perhaps it might like KPL instead.
I think your logic is quite correct.
However the MPG , as you know, is generated from the measurements of distance (Miles) and fuel used (Gallons).
You need to confirm that these two are (about) correct.
Because these two measurement are not directly available it is more normal to calculate these from fuel rate (Gallons per hour) and speed (Miles per hour). If you divide the Miles per Hour figure with the Gallons per Hour you get Miles per Gallon.
On the Laguna there is a self diagnosics mode for the instrument cluster. This is entered by holding the computer button on the end of the wiper stalk whilst pushing in the card to the ignition position. You can then see things like fuel flow rate and fault codes.
As I understand it, the fuel rate is sent from the Injection computer and the speed is fed from the ABS computer.
This is collected via the vehicle CAN bus so there may be a fault there.
When you exit the test mode it resets the faults.
Hope this helps.