60,000 miles of fuel economy, so what?

Being the data analyst that I am, I really get a kick out of seeing human behavior’s influence on data.  Explaining data anomolies is what I do.  So, recently, when my Ford Focus flipped 60,000 miles, I decided it was a good time to review the fuel cost and consumption projections that I put together when I bought the car six years ago (a topic of another post). However, when I charted the average miles per gallon (mpg) for each fillup, I got very excited.   The chart (below) had very distinct patterns and when I got to looking at the dates they mirrored changes in my own life. So, let me share a bit of my life with you and review the history of my car’s fuel economy.

Focus Fuel Economy

In the first part of the chart, marked A, there is a very nice sin curve. During this period I was commuting to college about 15 miles one way using the freeway as well as some city driving as I worked downtown.  It is interesting to see the drop in fuel economy during the summer of 2002, resulting from me not commuting and instead driving 4 miles to work on city streets. Of course, after school starts back up at the end of Sept-02, we see a rise in fuel economy.

The next major pattern change came in the months after my college graduation, marked at line B.  While the fuel economy does go down during the summer, this time it never goes back up. In fact, during certain times it gets far worse. So what was the change in behavior that drove this?  Easy, after graduating, I went to work full-time at my job downtown. This meant that most of my commute was on city streets for short periods as I seldom even got on the freeway.  The extreme drops in fuel economy are probably due to the times that I was house-sitting for a couple that lived at the top of a very big hill.

The latter half of the area marked B, represents a time when the car had not been serviced regularly. The lack of spikes during this time frame supports this as I would always have a spike in fuel economy after a service.  Now, moving into period marked C, represents a period of more steady service. The spikes during this time all correlate to a service interval.  I find it fascinating that these are all visible in the chart.  In addition, in Jun-04 I replaced the air filter for the first time and I started taking weekend trips and spending much more time on the freeway,  visiting places like Palmdale, Solvang, and Santa Maria.

The period marked D, beginning in Mar-05, brought about another change in fuel economy. This is when I got fedup with trying to make a living on the central coast with its skyrocketing cost of living, and moved to Texas. The major spike around 3/11/05 is due to driving to Texas with my most important possessions packed in the car!  Clearly, after arriving in  Texas, the fuel economy remained high. This is due to the location of where I was living; a rural “ranch” outside of town where the low speed, backroad cruising is excellent for sipping gas.  However, I was only in Texas for four months, returning to the central coast in Jul-05.

Immediately after returning to the central coast, I experienced a significant drop in fuel economy, period E. This is likely due to very short trips (I was out of work and mainly going to the store) and constant use of the air conditioner.

However, in  Sept-05, I replaced the stock air filter with a high performace K&N unit. This seemed to make the car breathe easier, so during the period marked F, I experienced a more stable, slightly higher fuel economy. Another change that likely impacted fuel economy during this period, is that I got a job and  started communting again. There isn’t much change during this time.

In January-07, marked G, I replaced the stock spark plug wires with a set from FocusSport (8mm versus the stock 4mm). This smoothed out the engines idle and also gave it some added power. However, I experienced a slight decrease in fuel economy. I think this was because I drove a little more aggressively as I found it fun to rev the engine.

The next change came in Jun-07, just before H, when I replaced the K&N filter with an OEM one and reset the computer.   These actions gave me a slight increase in fuel ecomony.

Following a service on 7/24/07, I saw a nice spike in fuel economy, marked I.  This was just before I left on a driving vacation to the Pacific Northwest. Between lines, there is a fairly massive spike that hovers around 40mpg for a few fillups as I was mostly drove on the freeway, cruising between cities.

It is interesting to compare this road trip with the road trip to Texas. When I went to Texas, I averaged about 32mpg, whereas when I drove to PacNW, I average 38.  The primary difference between the two trips is that when I went to Texas, I had many hundreds of pounds of stuff in my car, whereas the PacNW trip, I had a suitcase and some light camping gear.  This proves the point that weight can influence fuel economy.

To take my car’s last six years of fuel economy and make something of it, I think it is important to point out effects on fuel economy.  There are a number examples illustrated in the chart and narrative above.

1) Type of Driving; Urban vs Rural
Cruising at a moderate speed delivers the best fuel economy.  The start and stop of city driving can dramatically reduce fuel economy, especially the time spent idling at streetlights. One reason why the Toyota Prius gets 45+ mpg in the city is because the gas engine is not always on while the car is stopped. In fact, some hyper-milers have gone to such extremes to duplicate this by turning off their engines while at a stoplight (not recommended!).

2) Regularly service your car
The fluids that protect your engine and make it run smoothly can break down over time, causing excess friction. This friction can lower your fuel economy.  It is also a great time to make sure your tires are properly inflated.  Even 1 psi too low can cause your car to use more gas than it needs to.

3) Don’t carry more weight in your car than you have to.
Carrying more weight in your car means that your engine needs to put out more power to go the same distance.  This, obviously, drives down fuel economy.  Comparing the long road trips I discussed above, a few hundred pounds of personal belongings led to a 6mpg decrease in fuel economy.  Another way to get rid of excess weight is to regularly wash your car and have your engine cleaned every 60,000 miles if you experience any leaks.

4) Drive conservatively.
If you are constantly flooring the gas only to stomp on the brake at the next red light, you are wasting gas. Hypermilers pretend that they don’t have any brakes and avoid excess acceleration.  If nothing else, drive smoothly and don’t worry about beating the airhead in the Porsche to the next red light.


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