3.0 L engine and my gas mileage is at 15 liters/100km in the city of Calgary.
I use Ring Road to commute to work so I go like 130km/hr and it should not be guzzling like that.

 

How is your driving otherwise? Are you pin it to the floor and fluctuate up and down rapidly? I have the same engine in my S4 and when I first got the car it was averaging about 15-18L/100km. After a while and getting used to the car I ended up being able to average about 7-11L/100Km

 

I never press gas to the floor as it would go 200km/hr in an instant. I press maybe one/third or one fourth depending on the wind which is a constant factor in Alberta.

 

We have a 2005 3.0 V6 as well with manual transmission - at 100-110 km/hr I can get easily achieve 10 l/100km. That extra 20 km/hr will impact your fuel consumption a lot but the times I drive on the Coquihalla and am getting into the 120 km/hr range I still don't see that much of a drop-off. Being at 5000 ft altitude may account for some of you consumption (I'm usually down near sea level).

My suggestion is don't ever use cruise control, set the trip computer display to instantaneous fuel and for awhile drive like there is a raw egg in a shell between your foot and the accelerator. Not necessarily fun but you should be able to get a better feel for how light on the throttle you can be at a given speed. For short periods of time I will challenge myself to achieve 10 l/100km or better - really teaches you to look a long way ahead, stay off the brake and drive very, very smoothly with other traffic around.