It could be a couple of things. The sending unit (which is in the tank), or the gauge itself. Your sending unit works on a potentiometer, which moves your needle up and down in conjunction with your fuel level. The typical tank on the Subie’s is 15 gal. The gas light would turn on when 2 gal. of fuel remains (typical) giving you time to refuel without running out. One typical sending unit problem is usually where you know (visually) your tank is full, but the needle shows empty. Another is when your needle is flickering back and forth like a seismograph. However, from the way you explain it, I do not see a problem with your fuel gauge system, it might just be a vapor build up in the tank and then again, it could be something else. Your driving habits are also a factor on how fast that needle goes down. If your Subie is in perfect shape, and bone stock, fill it up and drive for 250 miles on your trip meter. If your needle does not go below half, there is a good chance it is a sending unit problem. For that you’ll need to take the tank off since you cannot access the sending unit from underneath the passenger seat like in most Honda’s. However, I recommend that you have a reputable dealer diagnosed it. Since fuel electrical systems are hard to diagnosed. Good luck.

 

Thanks SUBARU_TECH for the corrections. I was thinking of another car from one of my customers while I was typing this, same scenario, wrong car, my-bad. As for the fuel gauge problem, I don't think there's problem as long you average about 300+ on a full tank, assuming the needle goes all the way E. However, SUBARU_TECH is right, do it under warranty and save yourself the hassle. Sorry for the mis info.