Trip Computer Average Mileage Question

[Audial@aol.com]

I tried the search engine and found it virtually useless.  

Here's the question:

One of the small problems with my "new" Quattro is that the trip computer
reports the incorrect mileage.  Somewhere around 7mpg.  How does the computer
assess mileage and which sensor is bad?  

Tried to be brief, waiting for the Bentley....

Thanks,

Alex.