My gas mileage stated by the computer is down almost 40 percent overnight. I also noticed when the ignition key is on and the engine is off there is a faint squeal that comes from the top of the engine towards the firewall. I suspect an electronics, emissions or fuel richness issue. Any clues what to do to get back my MPG is greatly appreciated. The plugs have 50k miles on them but the car runs smooth with no missing. I have gone through a tank of gas and refilled at a different station – no mileage improvement. Two weeks ago the car got 24 MPG around town, now it is 15 MPG at best. Thanks in advance for your help!