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Discussion Starter · #1 ·
Lately I notice that my display seems to be displaying some erratic behavior, particularly on the 500 mile avg. It will made large jumps of 0.5 mpg up or down in literally a few hundred feet of driving. When it happens a tenth, then another a few seconds later, then another, and another, etc. Obviously it just doesn't make any sense that my 500 mile driving fuel economy average could be so much in just a short period of time. I've never noticed it doing this before until just the last week or so. Also, I've noticed that my car's calculated fuel economy is becoming less accurate as well. It used to be near spot on accurate. It was always just slightly under actual, but less than 1% error. Last fill up the error was more like 4%.

I'm trying to think of a single explanation for these findings. I considered the K&N air filter, the transition to winter fuel and maybe the stanadyne fuel additive I was trying out. It seems any of those three items might serve as a possible explanation for the increased error of the fuel economy when I fill up, but I can't really think of a way that any of them would cause the erratic behavior of the 500 mile average economy readout. What single unifying hypothesis could explain everything? Another think I was thinking about that I have never set my "trip 2" odometer so I can keep track of my lifetime fuel economy since the car was brand new. Could it be that the portion of the PCM responsible for calculating, storing and displaying fuel economy information is "overloaded" with information from me never resetting anything and thus adversely affecting it's performance? This also seems unlikely just because you would think the PCM should be able to keep track of the fuel economy for millions of miles without any problems if you wanted it to. I put the stock air filter back in the airbox, and stopped using the fuel treatment. Anyone else have any thoughts?
 

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I've seen jumps like that on the 25 and to a lesser extent on the 50, but never the 500. It's because of the mathematics of moving averages, nothing more, nothing less.
 

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Discussion Starter · #3 ·
I've definitely seen the 50 and especially the 25 do that, but not the 500 either. My 500 mile average is around 45 right now, and I was on a 30 mile trip today getting around 50 for most of it. In that short time my 500 mile avg did one of those things where it dropped something like 0.7 but then later suddenly gained close to 1.5 back.
 

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If your moving average is removing a real low number and replacing it with a very good number it might make a difference like that , or visa versa
 

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If your moving average is removing a real low number and replacing it with a very good number it might make a difference like that , or visa versa
I mentioned something about this in the diesel forum a while back. I think it's kind of weird how it does the moving average. I kind of feel like a weighted average would be better. You wouldn't see the big, quick swings. It would also prevent those times where you could be getting significantly higher MPGs than your average, but the average is still not moving up (and the opposite). To me the weighted would make sense, but maybe there is a reason they don't do that.
 

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It's documented as the most recent x miles in the owner's manual. This is the definition of a moving average.
 
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