The squealling belt can be isolated . . . have someone start the car while you listen to the engine (hood open, of course) . . . just listen and look. The one belt that is a real pain in the one on the bottom . . . on my '89 it is for the alternator, a real pain. Could be that it has road dirt or oil on it . . . causing it to slip and squeal. Belts are cheap . . . easy to replace except when in a tight place. Another thing which may affect the belt squeal is an alternator crapping out, becoming harder to turn, causing the belt to slip. BTW, anything dragging like this could affect the MPG . . . but a 35% drop is more serious . . . gotta be something else. Did you gap the plugs correctly? Tire pressure OK? Timing? If you are not mechanically inclined, I’d ask a mchanic to see which belt is making this noise and see if the alternator is “dragging” . Are you certai that the oil level was 2-3 inches? That seems like a lot . . . maybe you should check this out with a mechanic. Good luck! Rocketman