Well I just bought a 2006 G6 GTP with 55k on it (km) and I'm only getting about 500km a tank (70% highway) which is right around 11.9 to 12.4L/100km.
I was hoping to squeeze a few hundred more km out of a tank and was wondering if there are any mods people have done that increase mileage? i.e. on my grand prix putting an intake on and doing some minor exhaust work allowed the engine to breathe a lot better and actually gave me better mileage.
You'll get good mileage with little weight in the car (a 300lb driver will get worse mileage than a 150lb driver!), take stuff out of the trunk that doesn't need to be there, make sure tires are at proper pressure. I find 34psi is the best compromise between mileage and comfort.
I get around 500km/tank with mixed city/highway driving, and still have around 1/4 tank left over. If I use the auto-stick and hold gears in higher rpm then mileage drops to as low as 350km a tank. In the winter I always fuel when it gets to the half way mark, to reduce any condensation buildup as I park my GT outside. Straight highway I managed 8.1L/100km at 115km/h. Keep in mind that the EPA doesn't account for final drive ratios. They'll test the car with the tallest legs to show the better mileage figures. This is why the EPA states "Your mileage may vary". A base V6 has a 2.89 final drive or around there, where as my GT has the 3.29 final drive, which increases engine speed by 200-300rpm at any given speed in top gear on the highway. Naturally, it eats more fuel as the auto-stick transmission is more complex and saps more power than the regular automatic.
Best thing you can do cost wise, is to get a good high flow (Amsoil EA) drop in air filter, run synthetic oils if you haven't done so already, and try to lighten vehicle weight. On a side note, I noticed a difference in mileage, more so with my Ford Taurus when I lost 72lbs. Just food for thought. The cost of an exhaust and air intake will take the near part of a decade to pay for itself, and it won't necessarily give you better mileage, they're advertised to give better 'performance' only. If you keep speed low, 110km/h or less on the highway you'll see a mileage improvement with just a premium drop in air filter.