Author Topic: Fuel trip meter give high reading  (Read 2132 times)

BrummPopBang

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Fuel trip meter give high reading
« on: July 06, 2020, 06:24:58 PM »
The fuel trip meter on my 55 plate 1.4 Jazz gives a high reading ie it's typically showing 49mpg when in reality it's around 42mpg. Can this be adjusted / repaired?

My Haynes manual does not seem to have anything about this aspect of the fuel system.
Kind regards
Stephen Ford

Jocko

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Re: Fuel trip meter give high reading
« Reply #1 on: July 06, 2020, 06:44:15 PM »
This is typical of the fuel display, or as it is known on this site, the "Fibometer". There is quite a lot written about it on here. The smoother you drive, and the longer the journeys, the more accurate it gets. There is nothing you can do to change the calibration. I disregard mine and just calculate my mpg. I have a ScanGauge fitted to the car, and it is much more accurate as you adjust it every time you top up, but even that is not spot on.

BrummPopBang

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Re: Fuel trip meter give high reading
« Reply #2 on: July 06, 2020, 08:46:10 PM »
Thank you for those details. Well, at least that sorts that question...

Fibometer ;D yes. very good...
Kind regards
Stephen Ford

culzean

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Re: Fuel trip meter give high reading
« Reply #3 on: July 07, 2020, 11:07:13 AM »
As long as you know that the on-board average MPG readout is a bit optimistic you can allow for it,  I just use it as a useful guide to how I am driving,  I know pretty well that if I zero the trip and then drive the car like I just stole it my MPG is gonna be cr4p,  and it I drive smoothly and slot into islands, avoid hard acceleration try not to use brakes my MPG is gonna be a lot better.  I think they should just replace the readout with a green smiley face or a red sad face to show you 'hows my driving'.... I am not into decimal points on MPG.......... They are the equivalent of the 'rivet counters'  https://idioms.thefreedictionary.com/rivet+counter

First time I heard the 'rivet counter' phrase was at the Large Model show at RAF Cosford ( which is close to us ) where people put huge amounts of time building models of aircraft that could easily fly with a child on board,  somebody was saying 'that is the wrong propeller for that Mark of aircraft or some such remark' and someone behind them said 'come on, move along - rivet counters not welcome here' - really creased me up.....
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

BrummPopBang

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Re: Fuel trip meter give high reading
« Reply #4 on: July 07, 2020, 11:42:32 AM »
That's fair comment. I was hoping, with it being Honda, that the reading would be more accurate. But after the above comments and my measurements, that's clearly a vain hope.

We have "jobsworths" down here in the south. The local sport centre say our club members can run on the running track but not use the hammer cage for throwing. So I wonder how that plan will save lives? Hey ho....

Who can talk about "bean-counters"... ?  ;D
Kind regards
Stephen Ford

culzean

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Re: Fuel trip meter give high reading
« Reply #5 on: July 07, 2020, 11:52:48 AM »
That's fair comment. I was hoping, with it being Honda, that the reading would be more accurate. But after the above comments and my measurements, that's clearly a vain hope.

I think Honda are pretty high on the list of cars where claimed MPG is close to the actual MPG that can be seen in everyday use, and Suzuki tend to be near the top.  There are that many different driving styles that it is pretty much impossible to get a totally accurate readout without throwing more money at the system and having more frequent updating or 'sampling rate' - if you want to see how a new driving style affects MPG zero the trip first and then the system will show about every 10 seconds a huge difference between say going uphill and coasting downhill,  but as I said earlier after about 50 miles have accrued on the trip the rate of change is more subdued as the fuel used 'rate'  gets averaged out over a larger mileage by the computer.
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

peteo48

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Re: Fuel trip meter give high reading
« Reply #6 on: July 07, 2020, 12:03:06 PM »
The 7 mpg difference does seem a lot but that particular figure might be an outlier. I have had tanks where the difference between actual and computer is only 2 mpg but I have also had 5 mpg and it seems to average out at about 4 mpg optimistic.

I'd agree with Jocko that the longer the trip the more accurate the "fibometer" is. Other variables might include the petrol pump shut off and how empty the tank was before you filled up.

I use Fuelly.com to record my mpg on an actual basis (brim to brim as it were) but, if I wasn't so obsessive about this the computer would give a decent ball park figure providing you allowed for up to 10% inaccuracy.

Jocko

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Re: Fuel trip meter give high reading
« Reply #7 on: July 07, 2020, 12:30:09 PM »
I too use Fuelly.com but the figure beside my avatar is from spritmonitor. Same value, just different sources.

sparky Paul

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Re: Fuel trip meter give high reading
« Reply #8 on: July 07, 2020, 12:39:36 PM »
Thr fibometer has its uses, once you get used to the numbers you normally see, you can spot any unusual trends which might indicate an issue, such as a binding brake.

I'm going to throw a cat amongst the pigeons now...

The MPG readouts are only an approximation, most cars appear to over-egg the mpg to some degree.

On the other hand, where do you get your mileage figures to calculate brim to brim figures, or punch into Fuelly? Odometer mileages are derived from the speedometer signal, and speedometers are all calibrated to over-indicate to accommodate tyre sizes, tyre wear, manufacturing tolerances, etc. - by up to 10%. That means that odometers also over-indicate, and if you are using the odometer readings in any calculation, they will be out too.

edit: I had to edit the last bit out, I think I had a bit of a brainstorm
« Last Edit: July 07, 2020, 12:46:09 PM by sparky Paul »

culzean

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Re: Fuel trip meter give high reading
« Reply #9 on: July 07, 2020, 01:05:21 PM »
Thr fibometer has its uses, once you get used to the numbers you normally see, you can spot any unusual trends which might indicate an issue, such as a binding brake.

I'm going to throw a cat amongst the pigeons now...

The MPG readouts are only an approximation, most cars appear to over-egg the mpg to some degree.

On the other hand, where do you get your mileage figures to calculate brim to brim figures, or punch into Fuelly? Odometer mileages are derived from the speedometer signal, and speedometers are all calibrated to over-indicate to accommodate tyre sizes, tyre wear, manufacturing tolerances, etc. - by up to 10%. That means that odometers also over-indicate, and if you are using the odometer readings in any calculation, they will be out too.


I agree,  you would need to tow one of those surveyors measuring wheels behind the car and carefully measure everything that went into fuel tank to get a decent accurate reading...
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

sparky Paul

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Re: Fuel trip meter give high reading
« Reply #10 on: July 07, 2020, 01:44:25 PM »
you would need to tow one of those surveyors measuring wheels behind the car

I think that's a great idea. You could stick a handheld one out of the driver's window ;D

Jocko

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Re: Fuel trip meter give high reading
« Reply #11 on: July 07, 2020, 01:56:42 PM »
The odometer is way more accurate than the speedometer. The speedometer, must by law, never show less than the actual speed and no more than 110% of the actual speed so this is built into the display to register between 100 and 110%. The odometer does not have this error.
What I have found, with both my previous and current Satnav, is there is very little difference between the mileages displayed by both. Also, if you get directions from the like of Google Maps, and follow the route, the difference from the projected miles and the actual miles is minimal.
The calculated mpg has lots of errors built-in, such as calculation errors as well as how soon or otherwise the pump cuts off. The only way to get a reasonable measure of mpg is over a long time and several top-ups. My average of 55.4 mpg (to date) is over 110 tanks full and 40,000 miles.

springswood

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Re: Fuel trip meter give high reading
« Reply #12 on: July 07, 2020, 02:05:41 PM »
Another thing I find the mpg display useful for is it shows when the engine needs attention. For instance last November my mpg was dropping, then I remembered it was a year since I changed the air filter. The one that came off was disgusting and immediately the displayed mpg recovered.

Though I use Fuelly it only gives me a long term average. For some reason the same pump will cut off with wildly differing amounts of fuel in the tank. My 'brim to brim' figures have swung between 33 and 61 in the last couple of months.
« Last Edit: July 07, 2020, 02:08:36 PM by springswood »
"Indecision is a terrible thing"
Or is it? What do you think?

peteo48

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Re: Fuel trip meter give high reading
« Reply #13 on: July 07, 2020, 02:46:40 PM »
Thr fibometer has its uses, once you get used to the numbers you normally see, you can spot any unusual trends which might indicate an issue, such as a binding brake.

I'm going to throw a cat amongst the pigeons now...

The MPG readouts are only an approximation, most cars appear to over-egg the mpg to some degree.

On the other hand, where do you get your mileage figures to calculate brim to brim figures, or punch into Fuelly? Odometer mileages are derived from the speedometer signal, and speedometers are all calibrated to over-indicate to accommodate tyre sizes, tyre wear, manufacturing tolerances, etc. - by up to 10%. That means that odometers also over-indicate, and if you are using the odometer readings in any calculation, they will be out too.

edit: I had to edit the last bit out, I think I had a bit of a brainstorm

Good point!

peteo48

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Re: Fuel trip meter give high reading
« Reply #14 on: July 07, 2020, 02:48:14 PM »
The odometer is way more accurate than the speedometer. The speedometer, must by law, never show less than the actual speed and no more than 110% of the actual speed so this is built into the display to register between 100 and 110%. The odometer does not have this error.
What I have found, with both my previous and current Satnav, is there is very little difference between the mileages displayed by both. Also, if you get directions from the like of Google Maps, and follow the route, the difference from the projected miles and the actual miles is minimal.
The calculated mpg has lots of errors built-in, such as calculation errors as well as how soon or otherwise the pump cuts off. The only way to get a reasonable measure of mpg is over a long time and several top-ups. My average of 55.4 mpg (to date) is over 110 tanks full and 40,000 miles.

Even better point! My brain hurts.

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