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Sic Semper Tyrannis

Thursday, December 20, 2007

My Mileage Tracking Methodology

Aaron notes:
Make sure you start you log from an empty tank, then run on the same gas for a few fill-ups in a row.

If the numbers don't work out the way you want, make something up. That's what "they" do.
Source: See the original blog & read his comment for the source.

First off, my motivation for tracking is to see which fuel is the most frugal. So, I am not going to manipulate the results to come to my favorite answer (which is? The one that results in the lower cents per mile!), so don't worry no manipulation or data massaging to come to a particular answer.

Now, back in April I started tracking the mileage on our primary (and it was our only vehicle until a little more than one month ago) vehicle and got a very good handle on our gasoline expenditures and mileage. I think I have a good system down.

I fill up my tank, I take the receipt and write down the reading from my trip-meter and reset that. In addition, I write down the numbers from my trip computer (e.g. average MPH, average MPG, gallons used etc) for comparison. When I get home I plunk the numbers into a spreadsheet I built for just such purpose and get the answers.

I work on one assumption, the gas I pour in is the gas I burned to drive the mileage on the trip-meter. I am rethinking that assumption due to the nature of the experiment with different fuel classes (E85, E20, E10, and straight gasoline). I still believe my assumption to be valid.

Some years ago I did something similar to discern any savings by using high-test grades of gasoline. Since, high octane is universally available I drove exclusively one grade of gas for a a number of tankfuls. However, now I am not guaranteed E85 will be available at fill up time. BTW, I do not recall high-test making a significant difference just in cost. I am thinking about revisiting this one too.

The calculations I use are straight-forward from a seventh-graders math book. Take the miles driven, divide by the gallons of gas consumed, take the price paid multiply by 100 and divide by miles driven (cents per mile). In addition, I do some totaling and averaging (monthly & annual).

I take the gas down to about 1/16-1/8 of a tank and then I gas up (sometimes sooner, sometimes later I was getting low on fumes tonight).

Two interesting side-effects. First, you can detect problems before they become serious. A previous vehicle showed the first symptoms of transmission problems by decreased mileage and you begin to see how your approach to driving affects your mileage.

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