Gas mileage

Hey everyone. Have a question about gas consumption. I recently purchased a B6 1.8t Quattro, 5 speed.
I drive mostly highway (~80-90%) and it's been between 12-13 litres/100km, which is 18-19 mpg.  I use 91, which is the highest octane available in my area.
I don't drive like an old lady, but not like a 16 y.o either
Sure, there's always a difference between what they say on the label and the actual mpg, but in this case it's really a considerable difference.
Any insights as to what might be causing this. All the regular maintenance has been done, including filters. It has 100k on it.
 
Thanks in advance

Do u have an aftermarket rims on it???
Has an Oxygen sensor been changed???

Nope, the rims are original. As for the oxygen sensor, I really have no idea. One thing I know is that it's not showing any codes. Any thoughts ?? Plz .. ))

Are you calculating the mpg yourself?  Or are you going off of the display (it may be off).

Try some techron fuel cleaner.  Change the spark plugs (make sure they're gapped correctly).  Make sure your tires are inflated properly. 

13 liters per 100km is about 21 mpg.  I drive about 50/50 highway/city, and don't drive it too hard, and I only get about 23 mpg.  I don't think you're too far off the mark.

I have a Tiptronic and REVO stage 1, if I drive with a very light foot and no boost I can get around 27 ave on the display town and some highway. Normally, If I drive a little aggressively I'm doing around 20 on the display. Last refill I only got 20 for the tank but that was during the chip tuning which included a lot of dyno runs and me seeing how the car performed with the chip.  So I am getting less than I think I should too. I noticed 2 vacuum check valves that have white residue on them so I suspect they are leaking and I have them on order. We will see if that helps. Lots of things can cause poor mileage, tire pressure and tuning are basic things to check.