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Old 12-22-2009, 02:43 PM   #1 (permalink)
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Default My 100,000-mile MPG Curve

Yep, I'm one of those guys that keeps track of these things. I keep a book in my car and document every maintenance done to it, the date, the mileage, and I've kept track of each fuel up for the first 100,000 miles and calculated the miles per gallon per each fill up.

I purchased my 2006 N/A AT in April 2006, and it took me 3.6-years to get to the first 100,000 miles since I put ~65 miles on my car daily going to and from work. I've changed the oil at regular intervals, replaced my engine air filter every 10,000 miles, use practically the same brand of gasoline, performed the specified 30K, 60K, and 90K maintenance.

I've been entering the fuel data (date, mileage, calculated MPG) into an Excel spreadsheet and made an MPG vs. Time chart. My overall average is 26.8 mpg (), lowest monthly average is 24.0 mpg (March 2009), and highest monthly is I used 28.9 (July 2008). You can see seasonal fluctuations in mpg. The vertical lines mark each year anniversary from which you can see an overall yearly performance, and I see an overall decreasing trend in mpg. I can imagine it will stabilize at some time. </fingers crossed>

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Old 12-22-2009, 02:51 PM   #2 (permalink)
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Wow, that's really cool! I did that for the first 36k on my X.

Do you have a bigger pic? are the lows in the wintertime? Winter gas definitely effects mpgs.
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Old 12-22-2009, 02:58 PM   #3 (permalink)
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Wow, that pic IS small. I'll see what I can do to get a larger version.

And yep, the highs and lows correspond to warmer and colder seasons, respectively.
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Old 12-22-2009, 04:41 PM   #4 (permalink)
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Seeing the steady decrease in MPG and knowing the life expectancy of O2 sensors(roughly 100K with a slow steady demise), I would be curious to see a log of what you O2 sensor out put is.

Then see a chart for the next year with a new O2 sensor. Just my curiosity.
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Old 12-22-2009, 05:10 PM   #5 (permalink)
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Your chart is years from anniversary of purchase - if you arrange it by calendar year, you will see the seasonal pattern of heavu summer gas and lighter winter gas.

It looks like your gas supply for the whole of this year includes 10% ethanol, explaining why this year's results suck. (if you didn't want lousy mileage, you shouldn't have voted to have oxygenated gasoline?)

Sorry, sore point with me.
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Old 12-23-2009, 03:21 PM   #6 (permalink)
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Yeah screw ethanol. Good job President Bush. I have been using ethanol-free Shell ever since everyone else went ethanol mix.

Also, did you clean your fuel system at all?
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Old 12-23-2009, 07:16 PM   #7 (permalink)
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Very cool, but, sorry to see the decreased mileage! I've been noticing that too!
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Old 12-24-2009, 07:00 AM   #8 (permalink)
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I like the 02 sensor idea, I don't think you should be seeing such a drop over time...

for instance here's my 02 wrx mpg history (from april of 01 till end of nov09)

dm

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