Poor mileage

This could be due to a sensor issue which is telling the PCM that there is more air then there actually is and it is trying to match that with the proper amount of fuel. Or this is an over pressure issue from the pump/regulator.

Let's start there with checking fuel pressure. Here is a guide that will help with that:

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

Then we need to find out if there are codes. If we have a code that may help narrow this down. The idea would be the fact that this could be a misfire so hopefully we have some codes that will point us in the best direction.

https://www.2carpros.com/articles/checking-a-service-engine-soon-or-check-engine-light-on-or-flashing

Let us know what you find. Thanks