About 2 months ago we purchased a used 2001 Elantra wi/ 71K on it (now at 72800). The car was primarily a city vehicle driven by a college student for most of it's life, hence it's low mileage. After driving it for a month, I began to notice that gas mileage was erratic. One week we would see 29 MPG, the next week, 24.5. About 25 percent of drive time is city time, and since my routine is basically the same week to week I have a tough time figuring out what the cause of this is. Last Monday, it coded for a rear 02 sensor, which I replaced myself and I also did a full transmission service including a flush and new filter, followed by a full ECU reset procedure. After this, the car seems to run about 150-200 RPMs lower at a given speed than before. However, my gas mileage isn't agreeing with it. Prior to the work I could go 50-70 miles before my needle leaves the F on the gas gauge, and I only have 30 this tank and it's already leaving it. If this trend of erratic MPG continues, are there any other parts I need to consider checking, testing, and/or replacing? I have read that a MAP sensor can have a negative effect on gas mileage, as well as a clogged Catalytic converter (I find this highly unlikely given the mileage), and tire pressure. I have noticed no power loss or anything abnormal about how it runs, and the exhaust appears as normal. Maybe this is just some kind of reaction to the change in usage, since it went from being a city vehicle to a primarily highway vehicle, and I just need to give it more time/maintenance. I am also aware that the fuel blends this time of the year may take away a mile or two per gallon, and that higher ethanol contents can rob another 2 or 3 MPG. But am I missing anything else? Thanks.
2001 Elantra GLS