Bad mileage and a mystery puddle

[ProjectTCHG]

Hello all,

Seeking assistance with what I hope is one problem, might be two. I'm working on restoring the Celica in my sig, and I'm noticing I'm getting really bad fuel economy. I've owned the car since 2009 and used to get between 30 and 35 mpg, now I'm getting between 15 and 20. (car was parked for 9 months till the snow came back). No rough idling, no real noticeable lack of power, starts and runs smoothly.

I also have a mystery puddle that is roughly centered just behind the drivers seat. It appears to be a light viscous fluid with little odor. I'm starting to get relieved when I see the puddle, because it means I haven't run out of whatever I'm leaking.

I've checked through my Haynes manual and can't find any reference to the line directly above the puddle. My current guesses are transmission (manual transmission) or fuel.

Anyone have any advice?

[ProjectTCHG]

The leak is much worse while the engine is running. Whatever it is, it's a pressurized system. My money is now on fuel line. Has anyone done fuel line replacement before? I'm tossing between dealership and trying it myself.