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Old 09-17-2008, 01:00 PM   #18 (permalink)
EcoModding Apprentice
Join Date: Jul 2008
Location: N.C. / USA
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Unless you do've lost control...

Hi, all !
MPG is a shifting beast. So many factors affect this number.
If a ScanGauge (or any other such device) is added to your vehicle to keep track of this beast...this does not mean you can put away your pencil and paper. The gauge needs to be checked after installation before you can put any faith in it. All that is required is to keep jotting down the number of gallons per refueling / along with the odometer reading, and as the miles build up...check to see how close (or far apart) the two MPG curves are to each other. In a "perfect world", the average curves should almost overlap. In our mass-produced world, there will be a spread. This takes no effort...just a resolution to do it. It is not even necessary to fill up each time you refuel. Just start with a full tank ; note the gallons
( nearest tenth) pumped... and remember to use the last data point at the "point + 1" note - to get the mileage used correct.
Eventually, the total gallons used will approach a high accuracy ( for the odometer readings ( you did check the accuracy of your speedo via the mileage marker / time method, didn't you?).
If you really want to split hairs...the corrected odometer reading / gallons pumped method can be used as a "correct" baseline, and the calibration adjustments for the SG ( via the tire diameter / gas tank capacity pre-set parameters) can be adjusted ever so slightly to bring the digital ranges closer to the "correct" number. This is a pain, but it's better than not doing anything.
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