Hi all,
I thought that I would share some knowledge with you. Here is the way process analysis works (which is what we are doing with the fuel mileage tracking).
In order to establish a "base line" there needs to be at least 25 observances (fillups). Depending on how far you drive this could take a year
Once you establish your base line, then you can determine your "mean average" by taking all 25 fill ups, adding them together, then dividing by 25.
Once you get your "mean average" established, then there is what's known as a "standard diviation" (plus and minus). This varies by the observations but as a ball park figure is roughly 1.5% of your mean average.
What this means is that if your average over 25 fillups is 29.7mpg, then the standard diviation is 1.6, your range for milage is then 28.1 to 31.3.
If you get a milage figure that is outside the norm, (like for instance 40 mpg or 15 mpg) you need to analyze why that happened. Once you have determine why (high tail wind, 3 hour idling time, etc.) then this observation is removed from your running observations.
Another quick note, if you discover 5 or more points going higher or lower, then this is pointing to a "trend" and it would be a good time to analyze what is causing it.
Hope this helps
