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Improving Gas mileage (mpg) on a Prius

There are many things you can do to improve your gas mileage particularly if it used to be higher.

First if the check engine light is on get an OBD2 (On Board Diagnostic) reader and read the codes. They will tell you a lot about what is wrong with the car. The OBD reader plugs into a large D shaped connector at the bottom of the dash near the driver's left knee. The cheaper ones are less then $35 and give you less information but the information is still extremely useful. You just have to look on the Internet to understand the codes.

  1. Spark plugs. They may last 100 thousand miles but if you clean and re-gap them more frequently you will get another mile or two out of a gallon of gas. Any while you are looking at the spark plugs look at coloration of the deposits. There are lots of sites with pictures that explain this. Here is one.
  2. Air Filter just look at it. If it is dirty. Clean or replace it.
  3. Other general maintenance items. A site with lots of pictures for a Prius 2004 (2001 to 2009 have the same gasoline engine) is

  4. Replacing fuel injectors. If they are dirty, your fuel will not come out in a fine spray which rapidly vaporized. Plus the fuel quantities can vary from one cylinder to another. If it is bad enough, you will get a check engine light and a code. But your fuel mileage can easily drop 5 mpg (miles per gallon) or more before that happens so if your mileage has been dropping and you have 80,000 miles are more this is a good place to start. (The injector head pictured above came out of a Prius with 203 thousand miles. After the injectors were replace the Prius ran like new!)

  5. Driving habits: This is a huge subject but if you are racing up to red lights or are constantly heavily accelerating and braking you might want to change your habits. (Start slowing down as soon as you see a red light, if you time it right you will get through faster with a lot less fuel burned.) The gas consumption data on the Prius should help you.

  6. Tires: Put as much pressure in your tires as you are willing to and use low rolling resistant tires.

  7. Engine oil: It is easy to examine the condition of the oil by looking at the dip stick. Synthetic oil or oil additives will give you a few more miles per gallon less engine wear and longer engine life.

  8. Check engine air filter: Replace or clean if dirty. On the Prius there are three snap on and snap off clamps that hold down the top of the filter assembly it is very easy to access.

  9. Clean the airflow sensor. On a 2009 and earlier Prius it is a little square box with two Phillips screws. Directly below the air filter toward the front of the car. You can clean this with rubbing alcohol (found right in your bathroom) pouring it in and shaking it around. I also use a Q tip but if you do make sure you do not leave any cotton. You should also check the throttle body while you are at it. It is just below the air filter and can be access by taking off the air filter housing.

  10. Check spark plugs. If it has been over 100,000 miles replace them or clean and regap. They are right at the top of the engine and they are easy to access and replace; if you have a Spark Plug socket (on a Prius a 5/8 inch plug socket works) and 10mm socket for the bolt that holds down the ignition coils. On the Prius you do not even have to disconnect the wires to the coils and the wires are just long enough that you cannot insert them in the wrong cylinder. Inspecting the condition of the spark plugs tells you a lot about the condition of the engine. They should be light grey and not oily. The insulation around the center electrode should have some white. (There are internet pages with lots of pictures on reading spark plugs.) The gap will have widen after a long service life you can clean and re-gap or replace.

  11. Upstream Oxygen Sensor. The data from this sensor is used by the Engine Control Unit to regulate the fuel air mixture. If it is not working properly it can cause a 10 mile or more drop in MPG and generate OBD misfire trouble codes. Before it gets to this point, it can still lower you mileage. Unfortunately this is not easy to replace. You can monitor the voltage output of the sensor with the more expensive OBD readers. I believe my sensors started to fail at around 120,000 miles in my 2001 Prius. The upstream sensor used in the 2004 + are very different and may last longer.

    To replace the oxygen sensor in a Prius you need to remove the wiper assembly which is time-consuming but not particularly hard to do.

    The oxygen sensor is inserted in the top of the exhaust manifold just back of the fire wall. There is very little space to get at it and it is often seized in the exhaust manifold. If it is seized and you are using the correct wrench or socket and some penetrating oil and cannot loosen it do not waste a lot of time trying. (I have spent hours and all I accomplished was a rounded sensor!) Jack the car up and remove the manifold cut off the sensor just above the manifold. A small cylinder should fall out of the sensor making it easy to line up the drill but use a drill press and a vice to hold the manifold still. Use bigger and bigger drill bits till most of the sensor is out and then re-tap. The exhaust gaskets in a Prius are very high quality and I have reused them.

  12. 12 volt battery: Since on a Prius the 12 volt battery does not have to crank the engine (the high-voltage battery does, it just powers up the computers, lights etc.) it can degrade to the point where it lowers gas mileage and yet the car can still start.

    On a Prius you can check it easily right from the center console:

    For a 2001 to 2003 Prius go to

    For a 2004 + go to http://www.elearnaid.com/12vo1topraub.html#test

  13. Fixing or replacing the high-voltage battery. I have not had to do this yet so cannot offer a lot of advice except the pack can be taken apart and the individual cells are bolted together. Since they are in series you can rapidly see which cells are bad with a volt meter. More expensive ODB readers will also give you error codes that indicate the cells that are bad.
There can be many other things that will lower your mileage but the above list is some of the more common ones. All conditions will eventually deteriorate to the point where you get an error code and then the source of the low mileage can be easier to find.

Hybrid partsHybrid partsAs an owner of a Prius I have found some parts that you might want.(see below)

We have tested them on our Prius and been 12 volt selling batteries for hybrids since 2005. The Optima deep discharge batteries work great on a hybrid. (These batteries are chemically configured to allow more discharges instead of lot of cranking amps. You do not need cranking amps on a hybrid because the high voltage battery cranks the engine.)

"A short time ago I sent an email requesting the USPS tracking number. I am now pleased to inform you that this information is no longer necessary, in light of the fact that USPS has already delivered my Prius battery. In my opinion this is nothing short of miraculous! Kudos to you and your shipper, the U.S. Postal Service!

The battery was very thoughtfully protected within the Priority Mail box, by the way. Thanks for your expert assistance!

Sincerely, J. H., Clarksburg, MD

P.S. I ordered this same battery about a week ago, from Amazon, whose Marketplace at that time was defaulting to a competitor of yours. Four days later, I received word from them that the battery was not actually in stock at their warehouse, but would arrive sometime within the next two weeks. When I pointed out that their online inventory through Amazon Marketplace had indicated that it was in stock at the time of my order, and that it seemed like they could have informed me otherwise in something less than four days, they got quite a bit "huffy" about it (putting it mildly). Anyway, more to the point, in less time than it took them to tell me that their own inventory control was deficient, you actually put a brand new battery in my hands, and shipped it all the way across the nation to get it here. I won't forget it!

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