Real Life Mileage Vs. Dash Mileage

[oshoguno]

TL;DR - Dash mileage is inaccurate by 4%. That means it's off by 1.35 to 1.45 MPG.


Hi, I'm new to the site and new to my Civic, previously I drove a 2012 Focus from new. I recorded all of my fill up data religiously and being a bit of a statistics geek, I created a spreadsheet that tracked all sorts of mileage data. The results were weighted to include the distance traveled on each tank when calculating total averages, compensated for fill up inaccuraces (when it clicks off and how much you top off), tracked partial fill ups and compensated for them, and included graphs and plenty more bells and whistles. I got into a conversation one day with an Elantra owner who complained that he never got the mileage that his dash was reporting to him, always worse, his figured his dash was off by 0.5L/100km. So after the first 30,000 I started tracking the dash mileage with my fill up data. The results for the 2012 Focus were extremely accurate and within the margin of error. The dash mileage was optimistically inaccurate by 0.5-2%.

Overall Accuracy 98.729%
"The total difference between the dash mileage and the true mileage is 0.0852l/100km or 0.45 MPG. So the dash is reporting better mileage than what you are actually getting, so far that is worth $100.32. Since the dash reports to one decimal place and the actual mileage reports to many decimal places, the error factor is +/- 0.05 L/100km or 0.74%. Range covers 105,823km worth of data."
Total C$ $ 9,741.10
Total Liters 8884.43
Total Km 132,158
Best L/100Km 5.43 MPG 43.32
Worst L/100Km 8.55 MPG 27.50
Average L/100Km 6.72 MPG 34.99
Last 3 L/100Km 6.78 MPG 34.70

Now that I have the 2017 Civic with the non-turbo engine I have again started to collect data. It's very early and a really small sample size (just 4 fill ups) but it shows that the dash mileage is optimistically inaccurate by 3.25-4.75%.

Overall Accuracy 96.054%
"The total difference between the dash mileage and the true mileage is 0.2654l/100km or 1.44 MPG. So the dash is reporting better mileage than what you are actually getting, so far that is worth $6.35. Since the dash reports to one decimal place and the actual mileage reports to many decimal places, the error factor is +/- 0.05 L/100km or 0.74%. Range covers 2,282km worth of data."
Total C$ $ 159.00
Total Liters 153.49
Total Km 2,282
Best L/100Km 6.39 MPG 36.78
Worst L/100Km 7.20 MPG 32.67
Average L/100Km 6.73 MPG 34.97
Last 3 L/100Km 6.78 MPG 34.68


So if my dash reads 6.5L/100km then I actually got 6.7 or 6.8 which hopefully gets better once the engine finishes breaking in but should remain fairly accurate.

TL;DR - Dash mileage is inaccurate by 4%. That means it's off by 1.35 to 1.45 MPG.

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[turbo lover]

I assume that you are calculating your mileage from what the gas pump says and your odometer. You also have to consider that the odometer is not 100% accurate. Have you verified it against GPS or another more accurate source?

 

[derbo904]

Thank you for the TL;DR

 

[tsupersonic]

I've also had 4 fill ups so far. From my data, the dash reads 30 or 31 MPG (all city driving), though I'm actually averaging a calculated value of 29 MPG consistently. Like you, I've been keeping track via a spreadsheet (which includes dash mileage and calculated mileage).

 

[acurastohondas]

Just post your screen - mostly city and hills of Georgia

 

[tacthecat]

TL;DR - Dash mileage is inaccurate by 4%. That means it's off by 1.35 to 1.45 MPG. ...
So if my dash reads 6.5L/100km then I actually got 6.7 or 6.8 which hopefully gets better once the engine finishes breaking in but should remain fairly accurate.
TL;DR - Dash mileage is inaccurate by 4%. That means it's off by 1.35 to 1.45 MPG.

Our '12 Si Trip Computer is consistently 5-6% high - when we use the same pump and fill technique. It's still way better than the EPA numbers 32.9 vs 22 city/31 highway 25 combined!

 

[shire123]

Our '12 Si Trip Computer is consistently 5-6% high - when we use the same pump and fill technique. It's still way better than the EPA numbers 32.9 vs 22 city/31 highway 25 combined!

I've been recording this data on my 2017 EXT with MT since I got the car. 3 months of data so far. For me the dash figures are averaging being right on with the actual calculated MPG. Sometimes 0.5 MPG low, sometimes 0.5 MPG high. But never off by more than that. This was a pleasant surprise from my past HOndas which almost always were off by 5-7%.
So far in the 3 months my best MPG was 48MPG (on a 420mile fill with almost all 55mph highway driving) My worst was 41 mpg (on a 370mile fill with mostly interstate driving from Chicago-average speed 75, on a windy day).

 

[oshoguno]

I assume that you are calculating your mileage from what the gas pump says and your odometer. You also have to consider that the odometer is not 100% accurate. Have you verified it against GPS or another more accurate source?

Perhaps I'm not understanding you fully but how can the odometer be inaccurate? I know of no other system that would be any more accurate. Gps has an accepted Global Average User Range Error for coverage and then is influenced by signal blockage by trees, buildings, tunnels, covered structures, etc. (http://www.gps.gov/systems/gps/performance/accuracy/). Also, "URE is not user accuracy. User accuracy depends on a combination of satellite geometry, URE, and local factors such as signal blockage, atmospheric conditions, and receiver design features/quality." The odometer on a brand new car with brand new tires should be the most accurate it will ever be. I can see years down the road, wrong or different tire sizes, etc. that it might have reasons to be inaccurate. For now I'll trust the odometer.

 

[oshoguno]

I've been recording this data on my 2017 EXT with MT since I got the car. 3 months of data so far. For me the dash figures are averaging being right on with the actual calculated MPG. Sometimes 0.5 MPG low, sometimes 0.5 MPG high. But never off by more than that. This was a pleasant surprise from my past HOndas which almost always were off by 5-7%.
So far in the 3 months my best MPG was 48MPG (on a 420mile fill with almost all 55mph highway driving) My worst was 41 mpg (on a 370mile fill with mostly interstate driving from Chicago-average speed 75, on a windy day).

Were your dash readings always right or did they improve after the first month? My only problem with my data is how small the sample is, I'm not sure if the break-in period is throwing it off.

 

[glbg]

I was just done with my break in period , so time to get some mpg data. I filled up my tank last Wednesday, so next week I can get my mpg.

 

[syncro87]

Filled up yesterday. Computer said 41.7, hand calc 39.9 mpg. Diff of 1.8.

 

[cobrati]

I only have two fill ups so far, so mostly during break in, but it's been more accurate than my old RDX ever was. That thing would sometimes indicate 21-22 and I'd just barely hit 20.

In the civic, my two fill ups indicated 39.3 and 40.0 and actual was 38.3 and 39.95

 

[Videone]

My '16 trip computer has always been around 2 mpg higher than manual calculations.

 

[turbo lover]

Perhaps I'm not understanding you fully but how can the odometer be inaccurate? I know of no other system that would be any more accurate. Gps has an accepted Global Average User Range Error for coverage and then is influenced by signal blockage by trees, buildings, tunnels, covered structures, etc. (http://www.gps.gov/systems/gps/performance/accuracy/). Also, "URE is not user accuracy. User accuracy depends on a combination of satellite geometry, URE, and local factors such as signal blockage, atmospheric conditions, and receiver design features/quality." The odometer on a brand new car with brand new tires should be the most accurate it will ever be. I can see years down the road, wrong or different tire sizes, etc. that it might have reasons to be inaccurate. For now I'll trust the odometer.

There are sometimes odometer checks along the side of the highway that you could verify against. I would then double check those results against Google Maps and GPS. If all three indicate a bias in a certain direction compared to your odometer, then I would trust that the odometer is off. I know that there was a lawsuit against Honda a while back about their odometers being about 3% high. They are probably playing it safer now and it wouldn't surprise me if their odometers were under-reporting mileage now, especially with a new car with unworn and therefore slightly bigger tires. Under-reported mileage would mean that the fuel consumption display is more accurate than what you are calculating.

 

[xcivicx]

the official spec + & - is said to be "close enough for government work". which mean if you can go say 430 on the tank it don't matter. under most circumstances.
i always try to get 40mpg on trips (400+mil.). only a few time i could go that far with out having to stop, to get out and stretch or walk around. so my comps are close enough for me to not worry. never seen a next gas over 500 miles sign. maybe in alaska?

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