Hi!
Bought the P5 new, a little over 4 years ago, and I keep a tight record of milleage data. So here goes :
After 4 years, 114000km, total average is 8.48l/100km (27.74mpg US, 33.3mpg IMP). This average is summer AND winter driving. I've seen -40F quite a few times.
Yearly Averages:
In 2002 : 8.67l/100km (27.13 mpg US)
In 2003 : 8.34l/100km (28.2mpg US)
In 2004 : 8.61l/100km (27.32mpg US)
In 2005 : 8.36l/100km (28.13mpg US)
My very, very best in the summer was 7.38l/100km (31.9mpg US, 38.3mpg IMP), on a long highway drive.
I drive an average of 28000km/year, about 90% highway @ an average of 105km/h (Grampa style). Cruise control, manual tranny, Mobil 1 synthetic oil and K&N drop-in air filter. Summer rubbers are Kumho 711, winter rubbers when Jack Frost shows its freezin' nose.
First thing I noticed after just a few weeks after getting the P5 was that this car is quite a guzzler ( I had a 2000 Civic 4 door before) , and instead of getting better with time, it was actually getting worse, until I dicovered that the rear brakes were seized (always applied) in May of 2005. Now things are better (average since this major fix until today is 8.19l/100km).
My advice is this : have the brakes seriously looked at, and drive like on old fart.