Poor gas mileage

This normally means you have some blow by. Does the engine smoke at all? What about if you remove the oil fill cap when the engine is running, does it smoke?

Here is a guide on how to do a compression check. You may have one or more cylinders with low compression and if that is the case then we will most likely need to replace the compression rings in the engine.

https://www.2carpros.com/articles/how-to-test-engine-compression

Let us know about compression readings and we can go from there. Thanks