I have a 2003 Grand Am GT with only 40,000 km's on it and I can only get approx. 400 km's per tank! Recently, I changed the oil, the air filter, and the fuel filter, but have not seen a difference. I check the tire pressures regularly. I have not been driving the car hard at all. Those 400 km's are most highway. I have had a couple of other Grand Am GT's and they would easily get 600-700 km's per tank. What else should I do / replace to get better mileage. Other than the mileage, the car works great.
Thanks

You didnt mention which engine....

Could be your injectors are clogged/dirty.
Could possibly be your O2 sensors .

Might try replacing the plugs/wires if they are originals.

Also 400 kilometers (240 miles) is not bad for city driving .

My 99 SE w/3.4L(heavily moded) gets around 450-467km (280-290 miles) per tank(about 10 gal) in the city.
My GT 3.4 stocki, gets about 20mpg but it has a weak secondary O2 senor and I'm to lazy to replace it :p

Thier is a test procedure for the O2's here if you'd like to test them. Here is the link.. http://www.automotiveforums.com/vbulletin/forumdisplay.php?f=2148