When I first got my 08 mazda 3 with about 25,000 miles on it, I was getting 400 miles on every tank - averaging between 30-35 mpg (highest I ever got was 39 mpg, a couple of those days that week I was making long treks at 80-90 mph). 9 years later I'm at 180k miles and I get about 300 mpg now at best, averaging no more than 25 mpg. And actually, it's been this way probably for the last 3 years or so, at least. I have a hard time remembering whether this suddenly happened or if it was over a span of time.

About a year ago I switched to always using full synthetic oil with each oil change, though that hasn't seemed to help the mpg. I replace my air filter maybe every 20k miles if I'm being honest, perhaps I should be doing it with every oil change though. I keep my tire pressure filled as need be.

One website talks about a variety of things, some being potential bad oxygen sensor, fuel injectors, I've even heard of bad gas caps causing fuel loss. It also mentions spark plugs. I have never replaced a spark plug on this car ever. Nor have I ever replaced any belts.

Curious if others have the experience or any suggestions/thoughts.

 

The O2 sensor is the only one which will make any difference, and even then, only if your current upstream sensor is bad. The rest of the items, including things like air filter, will make zero difference.

 

Proper servicing, which covers air filter, fuel filter & spark plugs is a must on any car. Spark plugs are due every 120,000kms / 75,000 miles. Worn out spark plugs will cause a reduction in fuel economy.

Sounds like you car needs a really good tune up, which includes above mentioned parts a throttle body clean & maf clean & PCM/ECU software update.

When did the car last go to Mazda for a software update?

In regards to the O2 sensor, you will need to hook up a scan tool that displays live data ( PIDs) and with someone who understands the data, monitor both long term & short term fuel trims along with both the A/F sensor ( air fuel sensor or commonly called the upstream O2 sensor ) and the actual O2 sensor ( also called down stream or post cat O2 sensor). This data will give a good picture on how the engine is actually running.

 

With no engine check light to speak of and not mention a rough idle or start of idle stumble I would say spark plugs would be good first place to change out . Not sure why everybody likes to get so much millage out of them. Any one testing with an oscilloscope can easily see that spark plugs start to diminish their performance at about 20K-30k and begin even more demanding energy from the coils at a much sooner age/ miles than published and recommended to replace them by many sources.

I use standard Copper spark plugs and gap the plugs properly starting with the recommend gap and then experiment with larger gaps to find an optimal gap. They are less expensive and will give the same energy kernal as high dollar spark plugs, they possibly may only last 1/2 as long performing optimal as spark plugs with specialized component materials ? I have never tested a longevity of any spark plug, its a venture for the dumb curious IMHO!

The level of performance can also be seen and has proven time and again as good empirical data by using an oscilloscope instead of claims spark plug manufactures recomend that auto manufactures and even service intervals publish.

My personal preference are Champion 991 gaped at .046 using engine speeds not prolonged above 5200rpm But that just me spending time on my oscilloscope!

 

Sorry @Callisto we're going to need to agree to disagree. Modern double platinum and iridium plugs do not measurably diminish in their performance after only 20,000 miles; far from it.

 

Yes we can, but I did not read anywhere (also looked at search your user name) that you have ever done any testing or experimenting to support the blanket comment about spark plugs you gave?

Simply said for myself doing test examples on hundreds of spark plugs over the years comparing charts and to log scaled results in RPM ranges in milliseconds and the range of the coil energy demand at various spark plugs miles and age etc., concluded that a copper spark plug power gaped will produce an ignition kernal as good as any current none copper spark plug on the market, However I also suggested that copper spark plugs may not last anywhere near as long mileage perhaps from what I experienced about a little over half the life of say iridium. I have no reason to test the extreme limits of a wear pattern on a spark plug that can effect the performance out of an engine. Why would anyone? And I also mentioned that spark plugs are one of the parts DIY can change and effecting the most positive noticeable results. So it makes less sence to buy expensive spark plugs on the advertised HYPE about what they may be able to do instead of being frugal and buying a set of copper spark plugs and change them more often then the scheduled program that is for perfect engine wear operations.

If you want to see this in the real world........find a shop in your area that has a scope... make an appointment to do a 15 minute test run. Then simply change only one copper spark plug and see the results. PLEASE NOTE THE VOLTAGE/spark energy ON EACH CYLINDER AND WAVE PATTERNS and compare to the single cylinder with the copper spark plug.
I would also take notice that there is a limit to the spark energy that all MAZDA coils have as they are NOT a variable magnetic design.


Or ....contact any of the NATIONAL brand spark plug manufactures, copy and paste my last 2 responses send it to their technical departments and ask them point blank if anything I said was inaccurate and post it up.


I also want to mention I don't know to many car enthusiasts that do sanctioned or street racing that dont first change out their spark plugs before a race no matter how many miles are on the current set in the engine. No one likes the sound of an engine backfiring at high RPM because of the condition spark plugs used for a daily driving . LOL
I suppose you could cheat.... and blow some NITROUS in the engine at idle to clean them up....hahahaha!