gas gauge

It's in the tank. Keep in mind also that only the top two-thirds (roughly speaking) is actually gauged, so if you've actually got 1/3 of a tank left, the tank will show empty or close to it.

If it's showing empty right after a fill up, it's the sensor in the tank. I've never gone and done it, but if you use Google to search for terms like "Legacy" and "Gas gauge", you'll likely find some older threads of people who've actually done the repair. As I recall, you can either replace the module or clean it with some very fine sandpaper.

Thankfully you can get into the tank from inside the vehicle, so you don't have to drop the tank.

-Matt