You might want to bump up the rpm a little lugging the engine can be even worse on the mpg as running a high rpm

 

I'd try the upstream o2 sensor first. When I first got my truck it was gettin ultra ****ty mileage and would black smoke. Ended up being my o2 sensor.

 

I'd also check ur tire size...if they were changed and ur speedo wasn't adjusted for them that could make ur mpgs look worse than they are

 

I question your MPG calculation method.

You couldn't hardly burn that much fuel by just dumping it on the ground and lighting it.

 

I get bad mpg too. I only get 10 mpg on highway.Checked OBD reader all clean.
I used to get 20mpg new,but only have 23k on the truck now.
Starts great and run fine too.

 

well now wait a minute... you say you scanned the truck and it spits out a P\S MIL code? The fact that the check engine light is on will kill your gas mileage...

 

I took the truck to a mechanic last week and I'm picking it up today. He said the catalytic converter is a little dirty but not bad, but the throttle body injection was pretty dirty so they cleaned it up. Turns out I just had a loose clamp on the power steering. Changed the O2 sensor, plugs, air filter, and recently had an oil change. We'll see if there's any improvement when I get it back. Thanks for all the advice!
@Hahns5.2: 80 litre tank, 250 km per tank. Works out to about 7.35 MPG. I know, its bad.