Took my 2008 to the dealership for Spark plugs and Oil, now MPG has plummeted...

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?

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.

I was getting about 27 Hwy / 24 City before, and I've been checking it based off of the dash MPG readout (And I've reset it several times.)

 

I''m driving exactly the same as I was before, what's normal for these cars?

 

My main concern is that when they did the spark plugs / oil & filter change, that they may not have used the right plugs, or gapped them correctly. But could that cause this?

In my non turbo legacy I average 27 highway and 24 city, epa estimates for my car are 20 city / 27 highway / 22 combined.

 

EPA estimates for your car are 19 city / 24 highway / 21 combined.

 

So I honestly don't think there is any issue. I think the first few weeks were a fluke, and you are now averaging what you should be. Colder weather/winter blend gasoline could be a contributing factor as well.

Assuming you are using the cars MPG readings on the dash!!! When you reset the odometer trip A or trip B (typical after an oil change), the ecu resets the MPG reading on your dash board! This happens to me every 3k miles when I change the oil. The more miles I drive the more my MPG increases. I have had as low as 19mpg in the first 100 miles after a change (city driving) another time I've had as much as 28mpg in the first 100 miles. Basically the odometer needs miles stored in order to calculate an MPG!

Exactly what was said above. To the OP what is your trip A and Trip B odometer readings?

 

Also, I would think they would have disconnected the battery and you will have to let the ECU re learn all the fuel trims again.

 

The mileage should be back to normal if you have consumed a tank or two of gas.