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Gas mileage, Part 2,364,321.54a

I’ve had my 2003 Forester for 6 years or so. It had 18k miles on it when I got it and it has about 107k on it now.

I’ve always set the trip meter whenever I put gas in the car. Initially, I would very consistently be at about 300 when the needle hit the last 1/4 of the tank. I knew I still had an easy 50 miles or so if I needed it but generally always filled up at 300 miles on the trip meter and 1/4 tank on the gas gauge.

A few years ago, that number started dropping. Now, when the gas gauge says I’m sucking fumes, the trip meter says I’ve gone about 180-200 miles on that tank.

I’ve talked to my trusty mechanic…he’s done a tune up, replaced the fuel and air filters…I’ve done regular oil changes and maintenance. I keep on top of the tire pressure. I do have roof racks, kayak stackers, and an air deflector on top but I’ve had them the entire time I’ve had the car.

In a recent Car Talk show, they talked about the thermostat affecting mileage but I’m not having any symptoms of a wonky thermostat…the car heats up (temp gauge hovers right in the middle and I get good heat in the interior) within a few miles of starting out.

I’ve got no engine lights displaying either intermittantly or constantly. Does that rule out O2 sensor?

It seems to run smoothly…no stalling, hesitation or roughness.

What can I do? I really like the car but I’m really hating the mileage.

Although I have to admit that I don’t know what the actual mpg is (calculated from gallons of gas and odometer miles between fills) since I just go by the trip meter.

I suppose the trip meter could be wonky and I’ve actually gone more miles than it says when I fill. And I suppose I could have put a big dent in the gas tank that I don’t know about so that it now holds a lot less than it did. But those seem like kind of long shots.



How many gallons are needed to fill the tank? How many miles per gallon were you getting previously.

How about recording the number of miles and the number of gallons you used each time you fill it up, for several consecutive fill ups.  Then let's talk. 

 Gallons per tank or on board computer calculated mileage are often inaccurate.

If you are resetting your trip odometer every time you fill up all you have to do is divide the # of gallons into the # on the trip od. My guess, your ox sensor will set a light soon and you will kick yourself for not changing it sooner.

Do not ever rely on the dashboard fuel gauge for things like this. Compare the tripmeter to gallons required to fill it up and do the math that way.

The gas gauge sending unit is a variable potentiometer basically and it’s subject to wear. With wear generally one sees a higher resistance in the circuit. A higher resistance then artificially pushes the gauge needle lower than where it really should be, meaning that the needle may be flirting with the E mark even though there is a 1/4 tank remaining, etc.

Figure your actual mpg! You’ve beaten the subject into the ground attacking it from all angles and you don’t even know if you have a problem.

You say that you have performed all the maintenance that your car has, but what all have you actually done?

If your car has an automatic transmission, have you replaced that fluid, along with the front and rear differential fluid?

If you have a manual transmission, have you replaced that oil along with the rear differential fluid?

Chances are your gas tank is made of a plastic resin, so won’t dent.

When were the spark plugs last changed?
How often do you change your engine oil?
Maybe you have a brake dragging…

The list can go on and on.


It’s time to change the thermostat even if the gauge suggests otherwise.
I recommend a thermostat every 4-5 years on a daily driver.

Every time you buy gas, record the number of miles from the trip odometer and the main odometer. From this you might determine if the trip odometer is off.

Compare the quantity of gas bought to the tank capacity listed in your owners manual. From empty to full should be within 2 gallons of the total capacity. If for example, you have a 16 gallon capacity tank, about 14 gallons are usable. If you refill at the 1/4 mark, then you should be putting in about 10.5 gallons. The “you need gas” light should come on at about 12.5 to 13 gallons.

Another indicator is the frequency you buy gas. If this is your daily driver and you used to fill up once a week, but now have to fill every 4 days, that would be an indicator unless your driving habits have changed.

If you are really getting poorer mileage but no engine indicators, you could have a frozen caliper as indicated by outside pad wearing down much faster than the inside pad or a transmission problem. If a 5 speed, you might need a new clutch.