O2 Sensor and poor MPG's on 2001 GMC Sierra 2500

I have a 2001 GMC 1500 4.8l and I am getting 11-12 mpg's on the highway, could this be an 02 sensor? My air filter is new, plugs, wires, and fuel filter are also new.

Asked by for the 2001 GMC Sierra 2500
If you had an oxygen sensor causing that much of a fuel economy loss, your OBDII check engine light would come on based on a "lean" fuel trim issue.

A good diagnostic test is in order to see what is going on with your truck. A solid fuel economy test should be done. Make sure there is nothing causing an issue aerodynamically.
There is a check engine light on, I will shoot up to Autozone and have them check the code.

Thanks Dave!
OK, yes, check the check engine light - and if you have P0171 and P0174 together, you have a vacuum leak, air intake leak or something like that...which will make the engine ask for more fuel.
I checked the code today, it said engine cylinder misfire...
This too can consume more fuel, if fuel is unburned by a misfiring cylinder.

Or, worse yet, you could have a fuel injector on the misfiring cylinder sticking open. Check your oil level - does it smell like fuel at all? Is it high at all?

These are just thoughts. A good diagnosis is in order, otherwise it will be lots of guesswork. Good luck.
Thanks Dave, I will have too look into it a bit deeper.