Why does my gas mileage DECREASE after an oil change?

It seems like every time I change the oil in my car, I get WORSE mileage than before the change. What can cause this, and how can I fix it?

Update:

I'm measuring my mileage per gallon by dividing the number of gallons to fill the tank (at automatic shut-off) by the number of miles driven on the previous fill-up. I use the same octane fuel every time. It has happened every time since buying the car (new) that within the first 2000 miles after the oil change mileage is between 28 and 30 mpg, and between 2000 and 5000 mpg increases to 30 to 33 mpg.

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  • 1 decade ago
    Favorite Answer

    Should not be. There must be other factors, such as climate temperature change or vehicle has more load in it or driving over a different route. As far as the oil goes if you went from a 5w30 weight oil to a 10W30 oil then that could cause a slightly higher gas usage due to the higher viscosity of the oil which creates more drag on the engine. When your oil is dirty and at the point you need to change it chances are the viscosity is now lower due to contamination thus the less drag on the engine to operate, this could show up in the mileage. All in all the difference due to the oil alone is very slight and I would say very difficult to measure as a true comparison. As mentioned at beginning there are other factors that have a greater effect on the mileage. Gasoline octane is also a factor, hope this helps. Tire air pressure can be a big factor.

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  • 4 years ago

    No but I did notice a difference when I switched to synthetic 5W30 oil I use Quaker State Q Power I used to use Mobile one but Its never on the shelves. My car has better acceleration and I did see a 1 MPG improvement.

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  • 1 decade ago

    if its the kinda place that checks your air pressure for you while doing the service, they are probably letting to much air out of your tires is where i would start. ive seen many places that set the pressure to the "cold fill psi" not taking into account that your tires are warm from driving. they let air out, and your gas mileage suffers. i would check that first. if you do your own oil changes, it may be the weight of oil you are using. use the manufactures recommended weight for your vehicle.

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  • Anonymous
    1 decade ago

    millage multiplied by oil change equal to more fuel consumption.?

    millage. should increase if I never change the oil in my car?

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  • Lab
    Lv 7
    1 decade ago

    How are you measuring this, on a dyno with accurate fuel measurement, or guessing?

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  • duster
    Lv 6
    1 decade ago

    got me dude, millage should go up at least. what oil are they dumping into your mill?

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  • Anonymous
    1 decade ago

    are you acualy takeing milage/gallon with odometer...it shouldent change

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  • 1 decade ago

    maybe going to the wrong place .

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  • 6 years ago

    just had oil changed in new car. before changeing oil I got 27 mpg now it's at 22 mpg. Wondering if mechanic done something to my car to change the miles per gallon?

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