poor gas mileage

I suppose so but I think I'd check the fuel filter first. While you're checking it, replace it with the one you just bought, unless the filter has been replaced in recent memory, have a new one in your hand and install it rather than reinstalling an old one. If you can pour dirt and/or water out of it, it's no good anymore. If the fuel filter is clean and flows freely, Check the fuel pressure. Sorry I don't have a spec for you. I'd guess 35 PSI. Check the ignition system. Under load is when the ignition system will fail if it's weak due to bad plugs or wires, etc.