My trip mpg is out by roughly 10mpg for long journeys.

Actual MPG calculates as 35mpg but trip computer often reads 45mpg +

I have heard one can adjust the trip computer readings via VAG Com.

Has anyone done this?

 

Not sure if the trip computer is based on DIS, if it is you can change it

Adjust Consumption in DIS
[17 - Instruments]
[Adaptation - 10] -> Channel 3
change 100 to xxx (see formula below)

100 * DIS / Actual = xxx

DIS: 45MPG
REAL: 35MPG

100 * 45/35 = 129 (or 128)

Channel 3 will be changed from 100 to 129

 

This works very well....

Suggestion:

1. Fill up and RESET your trip meter AND (if you have them 1 and 2 trips)
2. Drive
3. On next fill up....fill LIKE YOU HAVE PREVIOUSLY (if you top off or if you fill to max...do it the SAME each time)
4. Drive
5. On this fill up do the same...resets, fill level, etc
6. DRIVE
7. Take your pen/paper readings and compare them to what your computer is telling you.

****Note: You need to be consistent in your fills and levels (to be the most precise).....once you have a "Pen and Paper" calculation you can really see what the computer is telling you.

8. LATHER, RINSE, REPEAT.........
--the more times you do this and manually record your settings the more accurate your consumption vs computer will be
9. ONCE you have evaluated your computer vs manual calcs...THEN you can adjust the settings via VAG/VCDS

Keep records. It is easy to write down prices, gallons, trip meter...etc

This will help calibrate YOUR car to what you are experiencing.