Visualizing 4 Years of Car Fuel Usage with Tableau (Part 2)

This post is a continuation of a previous post. If you haven't read part 1, you can go back to it using the following link:

• Part 1: Cleaning the data

Gas Price Trends

As you may recall from part 1, we've cleaned up the data and plotted the price per gallon as a function of time over 4 years, specifically for my hometown.

Price per gallon for fuel restricted to only refuelings in my hometown

Here’s what I notice:

1. Sometime in late summer of 2014, the cost of fuel took a huge dive that persisted for at least two years afterward.
2. There’s a seasonal fluctuation in the price of fuel with summer fuel costing more and winter fuel costing less.

The first point wasn’t surprising. Anyone listening to the news in the summer of 2014 is already aware that there were large price drops in gasoline around this time (just do a quick internet search and you’ll find lots of news articles about why this happened).

As for the second point, I was kind of already aware of this trend, but not really the reason. Apparently, this is caused by differences in the fuel blends produced in summer compared to winter. The reason behind these differences is a regulation by the EPA that restricts the type of fuel that can be sold in the summer as a means to reduce the level of smog in certain areas. The winter blend is cheaper, but it’s (supposedly) also less fuel efficient. Fortunately, I have the data to see if the second part of that statement is true.

Fuel Efficiency

Let's have a look at my MPG over this same time frame. No surprise, it's a total mess! What gives? I kept the same filters as I had in Part 1, but it’s clear that there’s quite a bit more spread in the points. To clean things up, let’s visualize a moving average where we average each value with the two before it and the two after it.

Miles per gallon (points) with moving average (line)

That cleans things up a little bit. And yes, there is a dip in fuel efficiency in the winter compared to the summer. So the fuel in winter is cheaper, but my car also uses it less efficiently. To be fair, there are many reasons why fuel efficiency might be worse in the winter compared to summer that are not directly tied to the quality of fuel, but I’m guessing quality plays some part. How big of a part? I can’t really judge from the above plot, but it looks like the efficiency doesn’t immediately change in September and May (when the fuel change-over happens). So I’m guessing the weather has a more dramatic impact on my fuel economy than the actual fuel itself.

Next Up

From a purely economic point of view, it’s worth asking whether the drop in price outweighs the drop in fuel economy. In part 3, I'll try to answer the question of whether the miles per dollar was significantly different in winter compared to summer.

• Part 3: Comparing summer and winter fuel costs