Alright so I've finally caved and am asking for help, my alero has 94000 km and for the past month I've been getting 260-280 km per tank wether it's premium or regular it's a 3.4l I'm not driving with a lead foot all ac is off, changed the plugs and wires, changed air filter, changed fuel filter, no trouble codes both o2 sensors are responding perfectly, cleaned out throttle body, new oil, tried fuel system cleaner that gave me maybe 10km more per tank, I'm at a loss everyone please help me out here.

 

You've covered most of the usual causes, except tire pressures and alignment. Other than that, maybe you have a brake caliper stuck, or the parking brake isn't fully released. Did the poor mileage just happen suddenly?
How about exhaust restrictions? Maybe your cat is plugged.
The change from winter to summer gas can affect mileage as well, if they do that in your area.