# Why is my gas mileage so bad??

My car is about ten years old and I have been noticing recently that it does not get nearly the same amount of gas mileage as it did when I first got it three years ago.  The first time I drove it to school, which is about a five hour trip; I was able to just make it on one tank of gas.  However, I went home for a weekend and when I came back I had to fill up about half way.  Even with my lack of knowledge with cars, I knew this was a problem.  What caused this sudden decrease?  I was due for an oil change, so I went and got that done.  The mechanic who did it told me that my tire pressure was at 28 psi and they were supposed to be at 35 psi.  With these two things fixed I'm hoping that I'll be able to make it home without having to stop half way for gas! However, are there other things that could cause a sudden decrease in gas mileage?

I learned that basically anything that has to do with the timing of the engine combustion has a lot to do with the efficiency of the gas mileage.  One of the main things that controls this is the oxygen sensor.  Having a bad oxygen sensor  can also lead to a "rough idle," meaning the engine has a hard time when it is just sitting at a stop light or something because a bad oxygen sensor will mess up the proper mixture of fuel and air, the timing of the engine, and the combustion of the engine as well. As far as I can tell, my car doesn't exactly sound peaceful when it is sitting at a stop light, to say the least.

How exactly does an oxygen sensor work?  Well, the engine isn't able to burn gasoline without oxygen; the oxygen sensor determines exactly how much oxygen is needed in order for the gasoline to burn with just the right amount.  The exact ratio of air to hydrogen to carbon (the elements found in gasoline) is 14: 7: 1.  If the oxygen sensor is bad, two things can happen; too much air can get into this ratio or too little.  Too much air is called a 'rich mixture' and there will be excess fuel left over after the combustion, which will be emitted into the environment and your car will not be considered under the limit of the 'okay amount' of gasses leaked.  On the other hand, having not enough air causes a 'lean mixture,' which can lead to bad gas mileage, amongst other things, and just bad engine performance in general.  A working oxygen sensor can judge when more oxygen needs to be let in or when less oxygen needs to be let in and it fixes the imbalance and it sends the information the car's computer which then adjusts the levels.

Having a bad oxygen sensor can decrease gas mileage up to 40%!!  "It is recommended my oxygen sensor manufacturers to get it replaced every 15,000 to 30,000 miles on older cars and at 60,000 miles for newer cars."  I realize that I have little to no knowledge about cars, but it's very alarming to me that having a bad oxygen sensor can decrease gas mileage as much as it does.  My car has about 175,000 miles on it, which means the oxygen sensor in my car should have been replaced about 6 times by now.  I can guarantee you that this hasn't happened!  Over break I am planning on having this done.

Is anyone really good with cars? What other things could be affecting my gas mileage this much?  Could it have been due to my tire pressure more so than the chance of a faulty oxygen sensor?

**All sources used are in the links within the blog.

this is a very helpful post for prospective car buyers. I wonder if this principle of the car's engineering applies to hybrid cars? would the oxygen sensor experience more technical difficulties if it belonged to a hybrid motor vehicle? also I am pretty sure other types of vehicles has this application with the engine so I wonder if the same recommended balance of oxygen is suggested for like boats or trucks

gr8 post.

My car has the worst gas mileage ever. I have a BMW from 2003. It drives fine but it runs out of gas so quickly. I always get so frustrated when I would have to fill it up all the time so I asked my dad if there was a way of fixing it to drive and have the milage that it used to have. My dad said that there was no fixing it and it's just an old, expensive car that needs a lot of gas to run well. The fact about the oxygen is very clever and something I haven't tried, but I also looked up ways to save on gas mileage and found many ways on this website: http://www.fueleconomy.gov/feg/factors.shtml. It says that breaking suddenly, driving quickly, and four-wheel drive are some of the many reasons your gas can run out quickly. Maybe before taking drastic measures to replace the oxygen sensors you can try these small things and see if it works.

Often enough the smallest things can affect the way that our car runs. The most common problem that people have is the same one you came across. The condition of your tires can greatly affect the milage that you receive. Your tires may have been in great condition but a small problem such as the pressure was causing you a big problem. Another thing many people do not realize wastes gas is going over the speed limit even at a constant rate. According to money USA if you are going 80 on the highway instead of the suggested 60, you could get the same gas milage as if you were on a road with constant stop lights. Also stated in the article is that you could lose milage for going to slow, and operating your engine at a lower rate then it is suppose to be used at. It is a fact that all of these reasons cause gas milage not to be as great. It would be interesting to see a blog comparing these in order to see if some are worse than others. In addition what can be done to increase milage and make your car more efficient?

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