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#2: 10-02-2013, 05:17 PM
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Originally Posted by ArmyMedic View Post
Just bought a brand new 2008 Subaru Legacy GT, and was averaging about 27 MPG the first few weeks

I recently brought it in for an oil change, and I decided to have them change the sparkplugs (at 60k miles) since I heard it would improve my mileage.

Now I'm only averaging 20 mpg... could the dealership have screwed something up?
I'm assuming you were driving mostly highway to get 27 mpg.

How are you checking your mileage? Normally you would fill up the tank, then drive X amount in miles, then re-fill the tank and divide the miles driven by the amount of fuel it took to fill up the tank.

Is the car running good? Normally, what causes poor fuel economy is, city driving (stop and go), heavy acceleration (driving habits), or possibly faulty O2 sensor.