Sorry, you didn't mention that the car only had 28k on it in your first post.
So, Yo checked the brakes, do you mean you took the calipers off and made sure those brake pads were not stuck in the caliper brackets? I see this a great deal and it will drastically reduce the fuel mileage on you.
It is possible that there could be an O2 sensor reading out of range, but most folks don't have a scan tool that can read the fuel trims and the O2 sensor outputs. If you do, please let me know and I will be happy to look at them for you. Since this site is geared more towards DIY folks, I usually try to lead with things you could actually do yourself that do not require $1500 scan tools. I can't just blame a sensor without a way to diagnose it, and I hope you understand why my original response was geared towards items you could look at in the driveway.
You may also consider using the air conditioning could be a factor. All these things add up to lower economy. a difference of 5 psi in tire pressures can have a huge effect. I mean from the tire pressures you had in the original tires.
I will try my best to help you, but need to know your expectations as well since I am not there to see the car, or look at these myself.