Visualizing 4 Years of Car Fuel Usage with Tableau (Part 3)

This is part 3 in a series of articles where I'm analyzing 4 years of data about the fuel efficiency of my personal automobile. You can find parts 1 & 2 Here:

• Part 1: Cleaning the data
• Part 2: Seasonal variations in fuel economy

Cost Per Mile

As a recap, the conclusion I reached in part 2 was that although the fuel sold in winter is cheaper, my car also uses it less efficiently. The question I want to answer in this post is whether or not the cost savings is enough to offset the reduced efficiency. Or, to put it another way, does the reduced price for winter fuel offset the fact that I have to refuel more often?

So what I want to get a feel for is the cost per mile in the summer vs. the winter. Plotting the distribution of "amount paid" per "miles driven" over time gives the "cost per mile", which I've shown in the following plot. Note, I've also plotted the market value of a barrel of oil over that same time.

Cost per mile and market price of a barrel of fuel

I can already hear you saying "well DUH, of course, the cost per mile is going to track the market value of a barrel of oil", but stick with me. The important take away from the above plot is that the cost per mile tends to be smaller in the winter than in the summer. So for the same amount of money, you can travel farther in the winter than in the summer. The difference looks like a 2-3 cents per mile, so that's probably about $10-$15 difference to fill up my 12-ish gallon tank in a car that gets about 30 miles per gallon. I had to fill up about once per week during this time, so that's around $40-$60 more per month in the summer time! 

So what leads to the fuel being cheaper in the winter and more expensive in the summer? This article provides a couple of ideas which I'll summarize here:

• The summer fuel blend is actually more expensive to produce:
  The summer blend uses certain additional fuel additives designed to reduce the amount of smog produced. Production companies also tend to shut down in the early spring for maintenance and to shift over to producing the new fuel blend.
• People travel more in summer than they do in winter:
  No surprise here. In the summer, kids are out of school and families go on vacation. This causes an increase in demand for fuel, which leads to an increase in price. Supply and demand, baby!

So in the end, the answer seems to be that the winter fuel is more cost-effective. But according to this 1999 study it's also potentially worse for your health and the environment.

Market Impacts on Cost

For the sake of argument, let's say I'm more concerned about the increased economic cost of fuel in the summer than its impact on the environment. Before I go to write my congress to change the EPA regulation, we might notice that the seasonal fluctuations are actually much smaller than the broader, more impactful market fluctuations. Or to visualize it another way, we can "bin" the data points by month. To better visualize "winter" vs. "summer" Tableau can also color the points by financial quarter: Q1 (yellow) and Q4 (green) are winter and Q2 (red) and Q3 (teal) are summer.

Cost per mile stacked by month and colored by financial quarter

It appears that the spread in the points over time is actually quite high. So what happens when we normalize the cost per mile by the market value of a barrel of oil? Well, we get the following image:

Cost per mile normalized by market price for a barrel of oil

"Why did the relative cost per mile increase, even though the market price for a barrel of oil went down?" The cost of fuel at the local gas station came down, but not by the same amount as a barrel of oil, hence the curve goes up instead of down. The peaks and valleys also appear to be sometimes in the summer, sometimes in the winter, and sometimes in between. So the market value of a barrel of oil probably has a more direct impact on the cost of fuel than the increased cost of summer fuel production.

Next Up

Do you have one of those dashboard displays that tracks your fuel efficiency and tells you your MPG? Have you ever wondered how accurate it is or whether it's slightly biased? In part 4 I'll start to look at the MPG reported directly by my car and see how that number compares to the raw MPG computed by <miles driven>/<gallons filled up>. I wonder how truthful my car is actually being?

• Part 4: Bias in car-reported fuel efficiency