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Old 01-10-2014, 02:14 AM   #1
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Ever wonder how accurate the dash MPG is?

A little history, I have kept track of every fuel fill-up since I bought my 2012 Focus SE Hatchback in Oct of 2012. Out of sheer boredom at work (I am in the maintenance department and sit around waiting for machines to break) I created a spreadsheet in Excel to expand on the information in my mileage app. I created running averages, sorted mileage by month, monthly totals for fill-ups and distance traveled, lots of stuff.

Recently I started to also record the mileage on the dash in my fill-up data, so if my dash says 6.7 l/100km and the app says 7 l/100km I can track the difference too. Thinking further I decided that comparing the two isn't good enough, it depends on how far you traveled on that tank, a weighted average if you will.

I have taken the calculated mileage and multiplied it by the distance traveled on that tank and added them all up. I did the same for the mileage recorded on the dash. So when I fill up and the tank gets filled a little more on one tank and a little less on another I can track the average.

What my calculations show me is that over the last 8 fill-ups (4383kms) my program says I have averaged 6.847 l/100km while my dash averages out to 6.752 /100km. This is factoring in the distance traveled on each tank so a tank that went twice as far as another one counts twice as much towards the average.

The results, for me anyways, is that my dash and the actual mileage differ by less than 0.1 l/100km over time for an accuracy of 98.6%. Without the weighted average of the distance traveled per tank the difference is 0.678 l/100km, that is if I just compared the dash and calculated mileage numbers.

Feel free to discuss my methods and results, but for me the dash mileage, over time, is really accurate. Next step is to compare the numbers for another car, my friend tells me his Hyundai Elantra is never right, figures his dash is always reading low by over half a liter per one hundred kilometers.

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Old 01-10-2014, 07:04 AM   #2
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I like spreadsheets.

If my commute stays the same, then generally my dash reads pretty close to my hand calculations. The only time I see any error worth caring about is when I go outside of my normal commute and take a trip or something (extended idling due yup child, etc.).

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Old 01-10-2014, 09:31 AM   #3
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Ive never kept track, especially the way you have but for the most part im within +/- 1 mpg every fill up from what the trip computer screen says. Same goes for my GF's focus.
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Old 01-10-2014, 09:38 AM   #4
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I used an app on my phone for a while to track actual mpgs. I also found the dash was fairly accurate over time +/- 1 mpg or so.
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Old 01-10-2014, 09:49 AM   #5
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dash = accurate enough for me, and I do keep track.
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Old 01-10-2014, 10:52 AM   #6
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I used Fuely and some other iPhone app to track MPG. Now I stopped tracking since my car gets such crappy MPG! I really gave up with it.....

Tracking it now just makes me more depressed! I don't even have it on my dash display. More concerned with the miles to empty display!
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Old 01-10-2014, 11:13 AM   #7
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I am in the habit of checking my mileage anyway. Used to just use the calculator app on my phone, now I use another app for keeping track. Generally, the dash is 1-2mpg off.
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Old 01-10-2014, 11:35 AM   #8
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My Fuelly average is 32.1 which is just about in line with what the dash says. Color me impressed, most cars I've driven can't seem to get that data right, off by at least 2 or 3mpg from actual.
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Old 01-10-2014, 12:17 PM   #9
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Mine dash reads anywhere from 1-3 MPG high on every singe fill up. It has never been accurate so I take it with a grain of salt. I use the Road Trip iOS app to track it and calculate it.
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Old 01-10-2014, 12:37 PM   #10
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Accuracy, at least for me, depends on how your trip is constructed. If I fill up right before a long highway trip (say 2 hours) then stay in the city for the rest of my tank, my MPG estimate is wildly off (shows 5-10mpg more than I get). However, if I fill up and drive around city and then do a long highway drive, it is usually within 1-2 mpg.

Basically if you "pad" the estimate with a long, nice and constant highway ride, it won't go down enough during city driving to show you a true estimate...but if you get your city mpg (usually pretty low) and allow the computer to even out with a long highway cruise, it'll be able to be more accurate...again, from my experience.
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