VW Golf Onboard computer reads low (Golf TDI mk4)

On my Mk4 VW Golf TDI PD150 I've noticed that the onboard computer fuel economy (MPG) is showing very low on the dashboard as part of the multi-function display. The dashboard seems to generally be showing around 15MPG lower than the actual fuel consumption.

Measuring the fuel economy by calculating miles travelled for fuel used gives a much better MPG compared to the computer reading low. The maximum MPG on the computer is around 40mpg yet the calculation from fill ups gives over 50mpg tank to tank.

Initially I thought that there was a problem with the car and that the TDI was using much more fuel than I was expecting. However after a fill up it appears that the MPG fuel consumption is around the expected amount and the onboard display is wrong.


VW Golf Onboard computer reads low (Golf TDI mk4)

I've not yet worked out where the MPG figure is calculated from but the good news is that the car appears to be returning the expected good miles per gallon that I've seen in previous VW Golf TDI cars.