## Gas Mileage Differences: Dashboard vs Calculator

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

 Table 1. Dashboard and calculated gas mileages for 26 tanks in a 2003 Honda Civic Hybrid.

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.

### Footnotes

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

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This article was edited after publication by the author on 10 Dec 2008. View changes.
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##### Huh? by Occams

only twice was the Reported Mileage higher than the Calculated Mileage.

That is not what your table shows. Do you have it the wrong way around?

It does appear to me that the dash display is exaggerating the result. I can’t see how your calculated method could be inaccurate provided that you are topping up full each time.

- although come to think of it, some cars do leak fuel through a breather pipe when filled to the brim. I used to have an old Datsun where it would even siphon out through a breather if you filled it all the way. This was not only expensive, but extremely dangerous. When I understood what was happening I made a habit of never topping up all the way.
##### Fuel Pressure Sensor by VnutZ

You need to use an OBD II reader and determine whether or not your vehicle supports a fuel pressure sensor. Only with a fuel pressure sensor can the ECU know ’’’factually’’’ how much fuel was actually used (based on fuel pressure and the injector timing. That value would then be tested live against the odometer for an absolutely accurate assessment of fuel efficiency.

Without that sensor, even the onboard computer must calculate an estimated fuel efficiency the same way the Scan Gauge II does – a hybrid of engine size, manifold pressure, RPM, air temperature and “fair guess” on the impact of ignition timing. If that’s the case, then the onboard computer is just giving an approximation and your fuel pump math is the correct value.

The fuel temperature thing was an issue last year, too, IIRC only the Canadians actually have temperature sensitive fuel pumps. [[http://www.omninerd.com/articles/Oil_Companies_Sued_for_Hot_Gas]]

##### I think it is the Dash by quad64bit

Some of your differences ‘5.93’ mpg would imply that at the calculated gas milage, the difference between the recorded and actual consumption is as much as a couple gallons: (5.93mpg*14.25gallonsConsumed ~ 84.44miles/36mpg ~ 2.346gallons)
note: I picked 36mpg since it is the median between recorded and calculated milage and I think its safe to say the actual milage is not more then the dash or less than the calculated.

My point is this; If the error was on the side of the pump (temp differences, innaccurate pump gauges, etc…) then it would result in an inaccuracy of approx. 2 gallons. I find this really hard to believe since pump accuracy is required by law, and an inaccuracy of 2 gallons in 14 is huge. Also, the change in volume of gas in relation to temperature is not great enough to create an effect that dramatic with such a small amount (<15 gallons). A quick google search yields the following: A 15 degree change in temp results in a 1% change in volume. So, to get a 2 gallon difference between what was pumped and what ended up in your tank, the gas would have to have been approx 210 degrees going into your tank, and then cooled to 0 degreesF to decrease 2 gallons in volume. (2/14.25 ~ 14%, 14%*15degreesF = 210 degreesF).

It also seems unlikely that your tank is siphoning off 2 gallons in the time period that gas sat in the tank. Another google search yields that pure gasoline evaporates at a rate of about 10cc per 57 minutes or ~ 0.00264172052 gallons per hour. This would mean that pure gasoline (which pump gas is not, pump gas has additives to inhibit evaporation) in an open vessel (which a gas tank is not, even the leakiest are essentially closed vessels) would take ~ 31 days to release 2 gallons into the air at 68 degrees F. Even if it were 100degreesF outside, the boiling point of gasoline can be as high as 400 degreesF depending on altitude and additives which makes this whole scenario unlikely.

Finally, it stands to reason the if the previous arguments hold, that the manual calculation is accurate. You cannot argue with division. Which would imply that your car probably takes the sum of a large number of pretty accurate estimates, but since that is essentially a Reiman sum, the accuracy is related to the difference in x where x is time. Most of the in dash systems I’ve seen only poll the fuel system once or twice a second. Therefor, to get really accurate results, the system would have to poll hundreds of times per second since the actual value = limit as x approaches infinity(sum(gasUsed/milesDriven)) for x.

Conclusion: Your dash is guessing :)

##### Here's a thought by Anonymous

If you sit idle, does the dash record your MPG as 0, and bring down your average fuel economy? If not that is why the dash gives a higher number than the calculator. The dash may only count your mileage when you’re actually moving where the calculators takes everything into account. That’s the assumption I made when I saw this with my vehicle.

-Chris

##### Scammed Again!! Down the Mighty Fall. by Anonymous

Whew. It appears you and the rest of us are indeed the victims of a miscalculation scam of some kind, but first of all let me say that if you use gasoline that has ethanol the gasoline burns up faster. We put it in our big 1989 Olds wagon and started burning rubber out of traffic lights. The darn stuff has more power but it also burns up faster. The flow rate or something must have been altered, which MAY HAVE thrown off your onboard computer detection.

I liked the other fellow’s comment about how your Honda system must be only showing mpg for when the car is moving. If true then your gauge is really showing you TWO THINGS and isn’t a scam at all, only by using a calculator able to see it. The gauge is giving you an accurate readout for MOVING MILEAGE in order to help learn the driver how to pump the accelerator pedal LESS HARD LESS OFTEN. The more you drive like a retired schoolteacher the better mpg you get. INDIRECTLY also telling you how much time you’re spending sitting in traffic, at traffic lights, etc.

So what you would really need to do is fill the gas tank and drive a straight 30 minutes to an hour on the interstate at 60 miles per hour then stop and fill the tank and compare what the readout on the gauge is showing to what the calculator tells you was burned.

I imagine the numbers will be a lot closer. But if there’s still a slight discrepancy I would then conclude the Honda Civic may not be calculating correctly and could in fact be from the amount of ethanol in your gasoline. The way to check that of course would be to fill the tank with non-ethanol gas and do the test again but for the entire tank this time.

It’s a shame they don’t do it and then tell all their car buyer’s the results because without being honest they end up losing Sales. People are getting really fed up with junk like this and then not being treated correctly when you brought the issue to their attention… because on the surface it really looks like to most people (who aren’t sleuths like you) that the readout was purposely set to show the highest possible number, making the Honda Civic User feel good so they’ll come back and buy another one later.

But if it’s just a matter of telling you the driver how much waste time you’re spending on the highway, that actually has value to let you know you need to find another route or time to do your driving. I’m a fellow sleuth like yourself and have been solving some right sticky energy & environmental issues for years. I finally in 2008 made a special page that contains my very best work. Give a visit sometime =>

…. ClimateEngine (Woody Riley)

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