The 1st gen 4Runners seem to have either the 17.x or 14.x gallon tank. The way to tell is by how far below the frame rail the tank hangs. The 17 gallon tank hangs about 3" below the frame, the smaller tank is about 1"-2" higher since it is that much less deep. My '85/SR5 has the 17 gallon tank, I have a buddy with an '85 base model and he only has the 14.x gallon tank.

Could also be the 17 gallon tank that is dented underneath, reducing it's volume.