Author Topic: On board mpg display.  (Read 301 times)

Jocko

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On board mpg display.
« on: January 13, 2019, 10:12:03 AM »
There is a lot of discussion regarding the on board mpg display, and how accurate or otherwise it is. I wanted to tell you about something I noticed, yesterday. When I started out my on board display was showing 51.1 mpg since last top up. My ScanGauge E indicates the average mpg for the current journey (as well as storing other trips, including the current tank). The ScanGauge E tends to be a lot closer to the calculated values, even though it gets its data from the same source as the on board display. This better accuracy is down to the fact that a correction factor is added (if required) at every top up.
What I noticed on yesterday's journey was that the on board had started to climb above 51.1 even though the ScanGauge was still showing less than that figure. In an ideal world, with accurate information, the on board would not rise until the ScanGauge was better than 51.1. In fact, it should have possibly dropped.
Even my calculated values show variation. I do my sums, as I have done since they started selling petrol in litres. I take the litres, covert them to gallons (to one decimal place) and divide that into the odometer miles minus previous odometer miles. It is how I have always done it. Fuelly, which I also use, does the calculation using litres without the conversion to gallons. That is more accurate and shows marginally better mpg than my calculation!
Even the miles can be inaccurate. If you use the trip miles they can be 425.4 miles but the odometer mileage would round that to 425. So mpg is all in the eye of the beholder and how it is calculated. Only thing it shows is whether it is better or worse than last time!!

MartinJG

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Re: On board mpg display.
« Reply #1 on: January 13, 2019, 11:28:39 AM »

I use two decimal points and then call it a day. I believe the actual conversion is 4.5460902819948 but life is too short for recurring figures which is what fuelly probably uses. I did pop over there to check it out and noticed it seems to be dominated by the folks from the US of A. There is a UK section but it seems their MPG is generally lower than us across the board. I know many of their 'Fits' have the 1500cc engine and maybe tighter emissions might account for this but I would have thought the greater distances would make for better average MPG. We all know the Imperial gallon is 'bigger' than the USA gallon for some bizarre reason. Have they done something funny with their American miles too? We do seem to live in an increasingly farcical world where virtual reality takes increasing precedence over factual reality.

culzean

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Re: On board mpg display.
« Reply #2 on: January 13, 2019, 11:37:31 AM »

I use two decimal points and then call it a day. I believe the actual conversion is 4.5460902819948 but life is too short for recurring figures which is what fuelly probably uses. I did pop over there to check it out and noticed it seems to be dominated by the folks from the US of A. There is a UK section but it seems their MPG is generally lower than us across the board. I know many of their 'Fits' have the 1500cc engine and maybe tighter emissions might account for this but I would have thought the greater distances would make for better average MPG. We all know the Imperial gallon is 'bigger' than the USA gallon for some bizarre reason. Have they done something funny with their American miles too? We do seem to live in an increasingly farcical world where virtual reality takes increasing precedence over factual reality.

Two decimal places is plenty,  it is easy to get too nerdy over MPG,  I know the onboard MPG display is not super accurate but I use it to show how different driving styles and weather affect MPG.  To me it is just a way to see a 'trend' in fuel usage at various speeds etc. On the occasions I used to do fill-up checks ( not recently as I don't do fixed commutes any more where terrain was the same every day and it was easier to see effect of octane / speed / driving style (controllable  ) and ambient temperature ( not controllable) on results.  I must admit nowadays I  just use it as a convenient display.

The MPG display is more 'real time' when the trip has been reset,  its response gets sluggish as miles get over about 50 to 100 as it is averaging any instantaneous changes in fuel usage over larger mileage,  it still takes a snapshot and recalculates and updates MPG display about every 10 seconds but the changes get smaller and smaller.

There should be an instantaneous usage display like a bar graph on the Jazz ( there is one on Civic) but I get my fingers slapped if I mess with anything on her-indoors car so I don't know as much about various displays as I do on Civic.   The display is hard to watch anyway while driving as it changes rapidly.
« Last Edit: January 13, 2019, 11:50:28 AM by culzean »
Some people will only consider you an expert if they agree with your point of view or advice,  when you give them advice they don't like they consider you an idiot

Jocko

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Re: On board mpg display.
« Reply #3 on: January 13, 2019, 11:46:57 AM »
I divide the litres by 4.54609 (it is done automatically in a spreadsheet), but the gallons figure I round to one decimal place.
In the US they use smaller gallons than us. They use an archaic "Wine" gallon instead of an Imperial gallon. In Fuelly you set your units to return mpg US, mpg UK, or Litres/100km.
On my posts I display all three figures, and currently my average mpg is 45.2 MPG (US), 54.3 UK MPG and 5.2 L/100km. As US participants display MPG (US) that is why their numbers are all about 20% lower than ours.


MartinJG

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Re: On board mpg display.
« Reply #4 on: January 13, 2019, 11:48:43 AM »

I use two decimal points and then call it a day. I believe the actual conversion is 4.5460902819948 but life is too short for recurring figures which is what fuelly probably uses. I did pop over there to check it out and noticed it seems to be dominated by the folks from the US of A. There is a UK section but it seems their MPG is generally lower than us across the board. I know many of their 'Fits' have the 1500cc engine and maybe tighter emissions might account for this but I would have thought the greater distances would make for better average MPG. We all know the Imperial gallon is 'bigger' than the USA gallon for some bizarre reason. Have they done something funny with their American miles too? We do seem to live in an increasingly farcical world where virtual reality takes increasing precedence over factual reality.



Two decimal places is plenty,  it is easy to get too nerdy over MPG,  I know the onboard MPG display is not super accurate but I use it to show how different driving styles and weather affect MPG.  To me it is just a way to see a 'trend' in fuel usage at various speeds etc. On the occasions ( not recently as I don't do fixed commutes any more where terrain was the same every day) and it was easier to see effect of octane / speed / driving style (controllable  ) and ambient temperature ( not controllable) on results.

Yes. Trend is the key for me. I am currently getting a reading of 45.4 on the fibometer which is down from @ 45.9 in the summer (from memory) which I think is better than expected after allowing for the seasonal change. Mainly country roads/carriageway. I do random real world calcs every now and then just to keep the fibometer on its toes and remind it of Queensberry Rules.
« Last Edit: January 13, 2019, 12:21:51 PM by MartinJG »

peteo48

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Re: On board mpg display.
« Reply #5 on: January 13, 2019, 12:51:58 PM »
I still do the nerdy stuff although I let Fuelly do the calculations for me. I don't really need to if truth be told because my small annual mileage makes fuel a relatively minor consideration but it's a habit I find hard to break!

In general terms I've noticed that, on longer trips, the on board display is more accurate in percentage terms. This must be because there are fewer variables in terms of driver inputs (accelerating, braking etc).


So, as long as you accept your real world mpg is actually a bit less than the computer says (between 5 and 10%) it is still a useful tool for comparison purposes and noting any changes as Culzean suggests.


But I'll continue with Fuelly - I am sad enough to enjoy entering the info and getting the result along with the graphs and pie charts.

Jocko

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Re: On board mpg display.
« Reply #6 on: January 13, 2019, 01:53:34 PM »
I am sad enough to enjoy entering the info and getting the result along with the graphs and pie charts.
Me too!

John Ratsey

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Re: On board mpg display.
« Reply #7 on: January 13, 2019, 08:31:18 PM »
There remains the question of why the "fibometer" is so inaccurate. The Mk 3 Jazz I had for a year worked out at about 9% optimistic while my current HR-V is only about 3% high. I have the theory that a bigger fudge factor was put in the Mk 3 Jazz calculation to help make the vehicle look more fuel efficient than its predecessor. The HR-V, on the other hand, had no immediate predecessor. There must be a fudge factor in the calculation as I've never seen anyone complain that their fibometer was under-reporting the mpg.

peteo48

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Re: On board mpg display.
« Reply #8 on: January 13, 2019, 08:56:45 PM »
That contrast in accuracy between your Jazz and your HRV is pretty dramatic John. Interesting.

Jocko

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Re: On board mpg display.
« Reply #9 on: January 13, 2019, 08:58:55 PM »
I have actually had one top up where the fibometer was lower than the actual value. Normally it is 1 - 1.5% optimistic.
The one time it read lower was a tankful last November. It was 447 miles and I added 7.3 gallons, giving me 61.2 mpg. The on board said 60.1 mpg, so not a huge difference.

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