Clubjazz - Honda Jazz & HR-V Forums
Honda Jazz Forums => Honda Jazz Mk1 2002-2008 => Topic started by: BrummPopBang on July 06, 2020, 06:24:58 PM
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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.
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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.
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Thank you for those details. Well, at least that sorts that question...
Fibometer ;D yes. very good...
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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.....
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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
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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.
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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.
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I too use Fuelly.com but the figure beside my avatar is from spritmonitor. Same value, just different sources.
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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
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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...
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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
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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.
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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.
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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!
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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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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.
If the car is on a slope - in any direction - the fuel cut off will be affected by that. If you are filling brim to brim and the list is the passenger side below the driver side ,then the fuel filler pipe will fill up before the tank - which will have air trapped in it... and thus contain less fuel. Ditto if the front of the is above the back...
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Just best to accept that there are inaccuracies in the system due to tyre wear and the fact that the tyre is flat at the bottom ( how flat depends on how much pressure inside, temperature and flexibility of sidewall ) maybe putting extra pressure in tyres like hypermilers do works because it increases tyre diameter and goes further per revolution of the wheel. Also speedo and oddometer get their data from same source = most likely the ABS rings on wheels, but speedo readout can be scaled to show a different reading to the incoming data.
I know the ambient temperature, humidity affect MPG ( anyone who has ever had a two stroke engine knows that they performed far better in cold damp conditions than hot dry ones due to density of charge ) but colder air is more dense so harder to push the car through it. I had a car once with a 'vacuum gauge' ( simply measured the vacuum in inlet manifold ) it was graduated from green through orange to red, the harder you pressed the loud pedal the further it went from the green area to the red, and you could sometimes hear a sucking noise from fuel tank area when needle was in red area.
Some people claim that driving slower to reduce fuel consumption means you will use more because the car engine is running longer to get from A to B, they obviously have no idea about physics because the fuel consumption is not GPM ( gallons per minute ) but MPG ( miles per gallon ) so time is irrelevant in the calculation - only mileage and amount of fuel used is important, and losses are less at lower speed ( that is why the record breaking Jazz that went Lands End to John O'Groats on one tank of fuel did 40mph the whole way ). To travel 2x as fast needs 4x the energy.
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It's interesting reading all the different views. One thing I had not thought of checking was for a difference between satnav distance and the odometer.
I fill up each time to the fuel nozzle and record that on an Excel graph. I had a Peugeot 205 1.1 years ago. The fuel usage graph was a very shallow wave from summer to winter (hi/lo) varying about 7mpg and with a ripple on the curve of about 2mpg. As someone else mentioned, a drop in fuel efficiency proved to be a warning of binding calipers. Just an aside - I fitted a set of Girling calipers in place of the original (useless) Bendix. The first time I tried the brakes I stood on them like I did with the Bendix. Hell's teeth - I nearly went through the windscreen...
But the curve for the Jazz is just crazy, up and down 39mpg to 55; all over the place - no rhyme or reason.
I hadn't heard of Fuelly.com and was surprised to see an average for the 1.4 2005 of less than 40mpg. At least mine manages 42. Although I suspect the front O2 sensor is dodgy (re: the above fuel graph) and have another to fit.
Just to add to the problems I removed the intake a couple of years ago and it seem I may have created a combustion problem. The plan is to work through all the issues but somewhere in all this I think I have a life...
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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.
I'm baffled now. How can the odometer be more accurate than the speedo if they are derived from the same signal? It used to be a single cable drive, but these days it's pulses from a gearbox sensor or ABS sensor. Miles driven is simply a function of speed and time.
There has to be some inaccuracy, to accommodate similar errors to the speedometer, and I'm not sure if it would be permissible for the odometer to read less miles than actually travelled...
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No doubt the speedo - which is electronic - has a formula to ensure it overreads..for UK.
The odometer I assume does not have one. (no legal need)
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No doubt the speedo - which is electronic - has a formula to ensure it overreads..for UK.
The odometer I assume does not have one. (no legal need)
Maybe not, but the signal is still derived from the same sensor as the speedo, with the same range of error.
Speedometers are designed to accomodate those errors, and still guarantee that they do not under read - for example, they may be calibrated to show 105%, to accommodate train errors of ±5%.
If the speedo has a range of inaccuracy of, say 100 to 110%, the odometer is still going to have the same 10% range of error as the speedo - even if that's ±5% on a calibration of 100%. What I'm saying is that the odometer may well be accurate, but it is just as likely to be affected by tyre size, tyre wear, etc., as the speedometer.
Does that make sense, or am I overthinking this? :D
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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.
I'm baffled now. How can the odometer be more accurate than the speedo if they are derived from the same signal? It used to be a single cable drive, but these days it's pulses from a gearbox sensor or ABS sensor. Miles driven is simply a function of speed and time.
There has to be some inaccuracy, to accommodate similar errors to the speedometer, and I'm not sure if it would be permissible for the odometer to read less miles than actually travelled...
Both odometer and speedo are presumably fed from the same sensors. If you think in terms of the old cable driven system the speedo can be made to over-read slightly (needle twisted slightly clockwise relative to scale)
Presumably in electronic systems there is some sort of electronic gismo converting the pulses to a display which performs a similar function of introducing a deliberate positive error in the speedometer. Or maybe the odometer just over-reads as well as the speedo?
Edit On second thoughts I think my second explanation seems more likely. As long as both over-read their would be no illegality.
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If the speedo has a range of inaccuracy of, say 100 to 110%, the odometer is still going to have the same 10% range of error as the speedo - even if that's ±5% on a calibration of 100%. What I'm saying is that the odometer may well be accurate, but it is just as likely to be affected by tyre size, tyre wear, etc., as the speedometer.
Does that make sense, or am I overthinking this? :D
Our last two posts overlapped. I think we are both over thinking this and that the odometer will be over-reading by up to 10% as well as the speedo.
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I think we are both over thinking this and that the odometer will be over-reading by up to 10% as well as the speedo.
Well, that's what I would have thought at the start of this thread... but I haven't done any scientific study, you understand ;)
My last point was, whether the odometer's target is accuracy, or is affected by the over-enthusiastic speedo reading, both must be affected equally by errors introduced by different wheel/tyre combinations, etc..
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Yes. I don't think it's going to make much difference if the odometer reading is slightly high.
The original post was about fuel trip meter being optimistic anyway.
That introduces a further factor - the petrol consumed.
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As noted earlier, there's a legal incentive to have the speedo reading slightly high so that the driver can't blame the vehicle (or manufacturer) if caught speeding. I recall checking my HR-V against my sat-nav and found it read about 2 mph anywhere between 30 and 60 mph. That looks to me like the speed being measured accurately and then 2 mph added on as a buffer. It would be easy for the electronics to use the TPMS sensor pulses.
I've not found significant odometer errors in any recent (or older vehicles) for the same approx 100 mile trip I've done intermittently for many years. It's a simple task to add up the wheel revolutions and multiply by the circumference and there's no technical or legal reason to include a fudge factor in the calculation. However, as the tyres wear then the actual distance per revolution is reduced so the reported distance increases as will the apparent mpg. A quick guestimate suggests that 5mm wear on the tyres will create an over-reporting of distance by 1.5% with corresponding increase in the calculated mpg based on fuel used and miles travelled. That would explain why putting new tyres caused a small drop in the calculated mpg for my HR-V!
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From my house to my mother-in-law's is 37.4 miles according to Google Maps. My odometer shows 36.8. My speedometer shows almost 10% over what's displayed on my GPS Speedometer and my Dash Cam, both of which read the same.
This says to me that the odometer on my car has a minus error and the speedometer a plus error.
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Just about every bike / car I've owned the speedo over-read and the odometer was spot on.
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Lightbulb moment. Or slaps forehead and calls himself stupid beggar.
I was thinking about the "computer" on my bike. When I set it up I had to input the circumference of tyre.
There is a pulse from sensor every revolution of wheel.
Distance gone = no of pulses X circumference of tyre and speed calculated by distance/time.
If circumference figure is wrong both distance and speed are affected.
In car if speedometer and odometer are regarded as separate a larger value of circumference could be input to speedo to give an inflated distance figure to calculate speed from.
Both speedo and odometer would be subject to errors due to differences in tyre size and wear but distance as measured by speedo would be consistently larger than distance measured by odometer.
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Both speedo and odometer would be subject to errors due to differences in tyre size and wear but distance as measured by speedo would be consistently larger than distance measured by odometer.
That was the 'scaling factor' I mentioned in earlier post, with electronics you can take the same information and apply a 'correction' ( ahem ) to it before it gets sent to destination / readout display. But there are still variables like differing tyre diameters, tyre pressure and wear as well as wheelspin that the ECU cannot account for.
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Thanks madasafish
If the car is on a slope - in any direction - the fuel cut off will be affected by that. If you are filling brim to brim and the list is the passenger side below the driver side ,then the fuel filler pipe will fill up before the tank - which will have air trapped in it... and thus contain less fuel. Ditto if the front of the is above the back...
What's puzzling me is it happens when I fill at the exact same pump which is on the level. Makes me wonder if there's something up with the EVAP system though I can't see anything.