I've recently done the SAT retrofit and I've updated the Kombi so it showns the correct display but since then, the MPG on in the display has been about 10mpg out.
My commute is 20 miles of motorway and dual carriageway so I know exactly what it should be doing and looking at the mileage left to refill, it's doing what I would expect.

So I need to change how it works out the MPG but how do I do that??