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

This post is the next in a series where I'm looking at 4 years of data from tracking the fuel usage of my personal automobile. You can see the first parts here:

• Part 1: Cleaning the data

• Part 2: Visualizing trends in fuel economy

• Part 3: Cost efficiency of fuel in summer vs. winter

MPG: Car vs. Real

Over the years, I've been in cars that had a display on the dashboard showing the fuel efficiency (MPG) over the duration of a trip. I always wondered how accurate those numbers were. My own car had one of these displays. So I tracked 4 years of what this indicator said about my fuel economy every time I filled up (resetting it every time) and compared that number to the actual miles driven divided by the amount of gas I put in. Here's what those two distributions look like. And just because we have the information, we can also see how those numbers change over time:

Fuel efficiency between March 2013 - Feb. 2017

Right away, we can see that over time the fuel efficiency of my car got worse! Dropping from the high 30's to the lower-mid 30's. This trend shows up in both the car-reported MPG and my own calculated MPG. This really isn't that surprising. Over time the car is going to wear and it's going to be less efficient as a result. This begs the question "what impact did this drop have on my wallet?". 

So, for a "back of the envelope" calculation, let's assume:

• my MPG dropped from 38 to 33 between 2013 and 2017
• I traveled about 17,000 miles per year
• A flat $2.50/gallon price as well (see previous parts of this blog that show this number fluctuates A LOT over time). 

Based on these assumptions:

1. I used about 447 gallons in 2013 and ~515 gallons in 2017 (difference = 68 gallons)
2. I sent about $170 more on fuel in 2017 compared to 2013

So, strictly based on the natural reduction in my car's fuel efficiency, I saw an increase of $170 in my fuel bill over the course of 4 years! That seems like a lot, but it's about the cost of a Netflix subscription for one year, so it's not extraordinary. Also, if I lived near where I worked, the impact would probably be a few cups of coffee different (actually, I'd probably ride a bike or take the bus instead and SAVE money). 

Another thing I notice is that the numbers reported by the car at least appear to be slightly higher. In other words, my car seems to think I'm getting better gas mileage than I actually am. We can do a comparison to see the bias in how my car reports the fuel efficiency of my car. We do this by making a histogram of the difference between the two MPG measurements:

The difference in car-reported and real MPG 

It looks like my car (at least initially) had about a 2 miles per gallon bias in it's reporting of MPG. Well, that's not great. But it also looks like, over time, the car becomes more and more accurate... what gives? Why did the bias drop from about 2 MPG to less than 1 MPG? Perhaps we're looking at this in the wrong way. It's possible that the car wasn't strictly 2 MPG bias, but rather it overestimated by a given fraction of the true value? Let's plot that distribution as "(car - real) / real":

Distribution of percentage bias in car-reported MPG

Nope, still shows the same trend over time. I can't really think of any other reason why the value became more accurate. There were a couple times when I had to take my car into the dealership for a recall of some sort, so maybe one of those times they changed the software that reported the MPG? Do you have any thoughts?

Just for completeness sake, we can re-verify the point I made back in part 2, that the MPG is seasonally dependent. We can do this in Tableau by coloring the MPG distributions by financial quarter (Q1,Q4 = winter; Q2,Q3 = summer):

MPG measures colored by seasons (winter=Q1,Q4; summer=Q2,Q3)

As expected, there appears a slight bias towards higher efficiency in summer and lower efficiency in winter.

Thanks for reading these posts. I think this will be the last part of this series for now. Did I leave anything out? Let me know in the comments and I can always add additional parts.