Ever since I posted the fuel economy versus E85 content graph, I have been thinking of ways to show the effect of E85 content on fuel cost per mile.
It is not enough to only report on fuel economy if the E85 is not cheap enough to offset the lower fuel economy that it gives.
Fuel cost per mile is the parameter that will show whether or not E85 blends are economical.
The problem is that fuel costs have been going up so fast it is covering up gains made by running E85 blends.
Another problem is that the difference in cost between E85 and 87 octane gasoline varies from station to station and with time.
So I have made a graph that reports fuel cost per mile while correcting for the varying cost problems.
The data points in orange are actual costs per mile resulting from the total tankful cost of different fuel blends divided by fuel economy.
The data points in blue are the actual costs per mile multiplied by the ratio of E85 to 87 fuel price, the ratio of the 87 octane price of the first fillup to the actual 87 octane price for each fillup, and the ratio of the average weighted tank fill price to the average adjusted tank fill price for all fillups.
I included linear and 2nd-order polynomial curve fits for both the actual and adjusted cost per mile.
The best of the curve fits is the 2nd-order polynomial for adjusted cost per mile, judged by the R-squared value shown with the equations on the chart.
The R-squared of each curve fit is not very good, so there would be a lot of variability around the actual calculated values from the equations displayed.
Perhaps the best conclusion that can be drawn from the graph is that cost per mile is pretty constant from 30 to 50%, then increases above that.
Have I earned the BITOG merit badge for CASSBAR? (Complicating A Simple Subject Beyond All Recognition)
