I have a lot of reasons to drive economically. It’s safer, I save money, and it’s better for the environment. Oddly, though, the thing that has made the largest impact on how economical I drive is having real-time and cumulative gas mileage readouts on my dashboard. Since I bought1 a 2003 Honda Civic Hybrid,2 I think I glance at the gas mileage readout more than I do the speedometer. The constant feedback allows me to participate in a constant competition to get the best mileage possible – a game, of sorts.
The “traditional method” of calculating gas mileage, on the other hand, doesn’t provide the real-time feedback. I still fill up and reset the odometer, but I don’t get more than vague indications of how economical I’m driving until I fill up again – when I can divide miles driven by the gallons needed to top off again. I might be able to recall certain aspects of how I spent the tank – like a lot of highway driving or having to run the air conditioning more than usual – allowing me to guess the reason behind the mileage, but I’d much rather be able to see the mileage drop four or five miles-per-gallon when I turn the A/C on or drive at 70 mph instead of 65 mph. I want to be able to know exactly what I’m giving up for the extra speed or comfort.
There’s more to the way these methods differ than just convenience, too. The first time I had a readout of my mileage for a complete tank, my curiosity got the best of me and I decided to see just how accurate the “traditional method” had been for me all of these years.3 The dash showed an average of 45.7 mpg over 509.10 miles. It took 12.13 gallons to fill up, so the calculated mileage was:
509.10 miles / 12.13 gallons = 41.97 mpg. “Wait,” I thought. “That can’t be right. That’s almost a 4 mpg difference!” 3.73 mpg to be exact, but I don’t think that takes away from situation: By the power of simple math, I had just taken the first step in proving one of three very important things:
- The traditional method could be fundamentally flawed, meaning mileage-conscious people everywhere are getting bad data.
- The Civic mileage readout could be flawed due to a CPU error (or, better yet, a corporate conspiracy), meaning owners everywhere are getting bad data.
- There are interesting nuances at play the likes of which I’d never before considered.
Investigation and Results
Ok, so the third option might not be that important, but it’s at least worth investigating – and investigate I did. Below is the data from 26 tanks of gas in my Civic Hybrid, roughly covering the first ten months of 2008.4 “Reported Mileage” is that shown on the dash, “Calculated Mileage” is from the traditional method, and “Mileage Difference” is the former minus the latter. Also, the gas station listed is the location where that particular tank was analyzed (i.e., the following tank is when that station’s gas was actually used). So, for example, Shell is listed next to tank 11, but Exxon gas (listed next to tank 10) was used while producing tank 11’s mileages.5
As you can see, that first tank wasn’t an isolated incident. The average of the Mileage Differences was just barely under 3 mpg, and this increases to 3.26 mpg when you average the absolute values of those differences. On three separate occasions, the difference was more than 5 mpg, and only twice was the Calculated Mileage higher than the Reported Mileage. Interesting.
Possible Explanations and a Conclusion
Although the investigation firmly convinced me there is, in fact, something fishy going on here, I must admit it didn’t rule out any of my previously posited situations. (The investigation really wasn’t geared at the why, though, so this isn’t much of a surprise.) I do feel a little better thinking about them more, though:
- Traditional method flaw – The main potential I see here for problems is with differences between the fill-ups. Although I stopped pumping at the first automatic cutoff each time, there could be sensitivity differences between pumps or nozzles. The temperature at the time of pumping could also have an effect on the actual volume of gas received.6 The traditional method doesn’t account for any of these, so one of the two inputs (the gallons used) could be seriously flawed.
- Dashboard display flaw – My Civic recently underwent scheduled maintenance and should be up-to-date on CPU fixes, but my investigation doesn’t do anything to remove the possibility of an unknown CPU error – or a massive corporate conspiracy.
- Other subtle nuances – This is really just an admission I could be missing something. (It wouldn’t be the first time.)
It’s also worth noting certain variables don’t appear to be contributing. The Mileage Difference doesn’t correlate with the date, so the air temperature shouldn’t be a factor. Also, while there aren’t nearly enough samples from many of the stations, there isn’t any indication certain gasolines result in a convergence or divergence of the Reported Mileage or Calculated Mileage. (Wouldn’t that be an odd trait for a gas to have?)
The best explanation, then, seems to be a flaw in the traditional method – obligatorily noting the potential for conspiracies, CPU flaws or unforeseen complications. If only I could convince someone on the inside at Honda to confess or investigate, and a few others to check the methods on other makes/models, I might be able to remove that asterisk.
In the meantime, you might want to think twice before punching numbers into your cellphone to calculate your mileage. I know you’re bored at the pump, but that’s no excuse to deceive yourself.
1 Hansen, Brandon. “Sometimes, a Hybrid is Worth It.” OmniNerd_. Accessed December 2008 from: http://www.omninerd.com/articles/ Sometimes_a_Hybrid_Is_Worth_ItIt
2 “One-Year Test Verdict: 2003 Honda Civic Hybrid.” MotorTrend_. Accessed December 2008 from: http://www.motortrend.com/roadtests/oneyear/ 112_0404_2003_honda_civic_hybrid/index.htmlhybrid/index.html
3 Yes, I’ve been calculating my mileage manually for years. Don’t act too surprised; I mean, the calculator was right there on my cell phone practically begging to be used.
4 The tanks recorded are not completely consecutive. If I remember correctly, there were two tanks for which I didn’t successfully gather data.
5 The table is available online as a published Google spreadsheet: “Civic Hybrid Mileage Analysis.” ‘’Google Docs’’. Accessed December 2008 from: http://spreadsheets.google.com/ pub?key=pC3mpnNSv5yLDyROGp7hpIQ
6 “Buying Gasoline in Arizona: Why Not Fairness at the Pump?” Unidentified author. Accessed December 2008 from http://www.users.qwest.net/~taaaz/AZgas.html #SOME%20NUMBERS%20TO%20THINK%20ABOUT