The dashboard computer reports abnormally low fuel consumption, possible solutions?

[WeekendDriver]

My actual fuel consumption is somewhere between 10 and 11 liters / 100 km (23 - 21 MPG), that's 90% city. Meanwhile, the dashboard displays low 8 l/100 km (30 MPG) values on average, and sometimes, when the traffic lights line up perfectly and I can maintain steady 60 kph, I get as low as 6.0 displayed by the computer (39 MPG). So, the computer reports erroneously low values. Can I fix it? What can possibly be the cause?

I've bought the car this summer and it was like this all along. It seems to run perfectly, there are no errors logged (checked several times across 2 months using VAG.COM), I've replaced the spark plugs and cleaned the injectors. There's no indication that there's actually something wrong with the engine.

[old guy]

Clicky click®

[WeekendDriver]

Thank you very much, mister! Now I'm curious to see if it's even set to 100 at the moment :)

[old guy]

Do you have a boost gauge? If so, how much boost are you running? Since your consumption is off as much as it is there is a distinct possibility that your B6 has larger injectors and a big injector tuning file. Some of the earlier BI tuning files didn’t include a correction for the calculated fuel consumption.

When I was running a GIAC big injector (386cc) file my gas mileage was overstated by 30%. I could only correct it to 15%. Now that I’m running a Motoza (550cc) tune with my F21L I am able to adjust the calculated mileage to where it is within ±2% of actual. Apparently Dave@Motoza includes an adjustment to the calculated mileage in his tuning file.

[WeekendDriver]

An interesting thought, but no, it's a bone stock BFB engine with no mods whatsoever, which was part of the reason I chose this car.

Do I understand correctly that the computer calculates consumption by multiplying the injector activation time (which the ECU itself specifies in the first place) by the pre-set, hard-coded injector performance rating (cc)? But then, bigger injectors would yield understated values, not overstated as you said was the case. I'm confused.

[old guy]

The first thing to consider is that the 1.8T fuel delivery is a “returnless” system and the fuel pressure remains constant at 4 bar. There is no rising rate fuel pressure regulator to account for boost pressure increases. It is all controlled by the injector duty cycle.

Gas consumption is calculated by an expected amount of flow based upon the injector duty cycle. When you increase the injector size you decrease the duty cycle for the same amount of fuel delivery. The ECM looks at the reduced duty cycle and assumes you are using less gas than you really are using.

[WeekendDriver]

Makes perfect sense now, thank you for the explanation.