My gas gauge is lowering, poor mileage?

The #1 reason for this to happen is a bad MAF (mass air flow) sensor, I would purchase a new unit (Motorcraft) and install it. Also, check the air filter as well and replace it. Here is a guide to help you with the repair and instructions in the images below to help you with your particular car:

https://www.2carpros.com/articles/how-to-replace-a-mass-air-flow-sensor-maf

Don't try to clean the sensor because once it gets to this point cleaning will not help. You should also check the air in the tires.

https://www.2carpros.com/articles/how-to-use-a-tire-pressure-gauge

Check out the images (below). Let us know what happens and please upload pictures or videos of the problem.