I have a 2008 Scion XD which crossed 126K miles today.(Bought it with 62K) In recent months I have noticed a decrease in overall MPG to a tank. I do close to 30K miles per year and also do oil changes every 5K with mostly highway driving. My question is where do I start on the maintenance to keep it going cause its a good car I just don't wanna lose out because of bad upkeep.

Other details last transmission fluid change was at 83K miles, tires are all new and the front brakes are fairly new I got them completely redone in December 2013. Other than its been oil changes and tire rotations.

Let me know if you need any more details.


  • How much of a difference per tank? Do you live in an area with seasonally blended fuel, some areas use a different blend in winter than summer to adjust atomization.
    – mikes
    Commented Mar 12, 2015 at 22:05

1 Answer 1


Where you need to start is at the O2 sensors and spark plugs, plus a thorough cleaning of the Mass Air Flow (MAF) sensor and upper intake tract.

While looking at the scheduled maintenance guide for your vehicle, I'm not seeing these things mentioned (though I did a cursory glance). It is common practice for original O2 sensors and spark plugs to last for 100k miles on a vehicle. I noticed you didn't mention you changed them, so am mentioning. What happens is exactly as you describe as these items become older and well used ... that being gas mileage slowly goes down. The O2 sensors become what is known as lazy. Basically, they don't work as efficiently so fuel mileage suffers. As spark plugs become old, they to do not burn fuel as well as they used to. The gaps become wider and wider until they fail to allow spark to pass. Changing these two things out (all spark plugs, plus front and rear O2s) along with a computer reset should help increase your fuel mileage back to where you are used to seeing it.

While you are getting them changed, getting the upper intake tract and sensors which affect engine efficiency cleaned will help as well. I would get this type of cleaning done every 50k to help with the optimal running of the vehicle.

Also, I would change out the air filter if this is something which you have not done. This can cause a lack of air flow into the engine, causing you to push further on the gas to get the same amount of power out of the engine. This will cause a reduction in gas mileage.

  • Thanks a quick talk with the shop said the same Spark plugs and O2 sensors. I will also ask about the air filter and sensors as I was unaware of those before. thank you very much.
    – Femi
    Commented Mar 13, 2015 at 20:29

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