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Sudden MPG drop, no change in driving habits or gas

My 2008 Honda Fit has been getting 32-34 mpg city and 37-38 mpg highway, with the AC running. Suddenly I have two tanks full that clocked in at 24 an 27 mpg, for mostly highwat driving. I have not changed type of fuel. I have owned the car since April 2011 and had consistant mpg driving everywhere from NY to FLA.

What should I have the mechanic check out first? I have a full warranty, but want some guidance before I just take it in and say "Fix it." I just turned 63000 miles.
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Comments

  • edited June 2012
    You may need one more tank to be sure this isn't an issue of the pump shutting off at a different time. However, let's assume you're correct.

    Is your tire pressure low? After a long drive, is one wheel much hotter than the others (in which case that brake is sticking on)? Do you have a check-engine light on? Are you up-to-date on all of your maintenance? If this is an automatic, is your car properly shifting into the highest gear, with the torque converter locking up? Are you using the proper grade of oil?
  • If the CEL is on have the codes checked and it will probably lead you to your problem. Such a drastic drop in mileage is often associated with a bad 02 sensor. I've had them go bad and cause a 25-35% drop in fuel efficiency.
  • Did you have anything done to the car just before the mileage dropped? A drop like this has to have an assignable cause, and it often traces back to a maintenance that was just performed.
  • The first thing to have checked is a stuck open thermostat. If the engine doesn't come up to full operating temperature the computer will believe the engine is still cold and stay in the open loop mode.

    If the thermostat checks out okay, then the next thing to check is a defective coolant temp sensor for the computer. If the sensor has failed where it's telling the computer the coolant never gets to operating temperature when it actually does, the computer will believe the engine is still cold and stay in the open loop mode.

    In both of these cases the engine will use more fuel.

    Tester

  • Check it a few more tanks of gas. It could just be a fluke. If the problem continues, the first thing is to bring all your maintenance up to Honda specs for your car. Look at your owner's manual for the specs.

    And think of what has changed recently. Have you driven in extra-dusty areas lately? A clogged air filter for example could cause this.

    After that, ask your mechanic to show you any stored codes.
  • If you are using the Honda computer to determine gas mileage, don't depend on it. Reset the trip odometer to zero and record the mileage when you fill up next. Using that and the gas volume on your receipt, you can determine the gas mileage accurately. I don't know how you determined gas mileage and thought I'd mention this if you do it the other way.
  • One low tire can kill fuel mileage so tire pressure can be a consideration.
  • You should be getting about 29-31 MPG overall with this car. If it was just one 27 MPG tank, then I would chalk it up to an anomaly, and it would still in the ballpark. But 24 MPG is a bit low. As the others have mentioned the methodology used in determining fuel mileage may be a factor here. Has anyone else been driving the car?
  • I would also try another gas station, or different gas. It possible your station just switched suppliers who are using a bigger blend of ethanol.
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