Mpg meter off by 4mpg - Page 2 - Yamaha FJR Forum : Yamaha FJR Owners Forums
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post #11 of 91 (permalink) Old 03-01-2019, 09:36 PM
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Quote:
Originally Posted by rubiete View Post
Quote:
Originally Posted by the Ferret View Post
Mine is off by a couple mpg when using a calculator, but I don't worry about it. In fact I quit checking it a few weeks after I got it. I don't even look at calculated mileage or current mileage. I go by mileage on trip 2 and I know it will go 220 + miles before the last bar starts blinking and I should be able to 50 miles or so beyond that. I just reset trip 2 every time I get gas.

It's a computer so i don't know how you could rectify it to come out exactly at your miles ridden/gas used/average mpg.
Thanks Ferret!! the thing is I can understand 1 or 2 mpg but 4...it is a little too much. will continue digging on this. Thanks!!

Well, good luck with that. It's an estimate and it is (like cars) based on a best guess at the time. Without actual fuel flow data.

The only way to be accurate would be to have an actual fuel flow meter. Which no manufacturer would pay for.

So the only choice would be to reprogram the computer logic and adjust whatever algorithm is used to "estimate" what your mileage is to account for how you ride in relation to whatever they considered a "normal" rider at the time.

And considering that they have to play games with the government regarding co2 per "whatever" per mile etc etc etc, it is a losing battle.

So, I applaud your tenacity.
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post #12 of 91 (permalink) Old 03-01-2019, 10:15 PM
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I would assume that the instrument panel is not of NIST calibration gage quality which needs to be at +/- 1% of full scale or better to meet that requirement so that aside and being that itís not calibration gage quality it is in the +/- 5-6% of full scale range. I would say that the avg. fuel economy meter probably goes to 100mpg so the accuracy would be +/- 5-6mpgís. Thatís going by normal gage accuracy standards and assuming it is manufactured to at least those standards, but thereís no garantee to that. Could be better but probably is worse, itís all a matter of $$$.

Thatís my swag at it.

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post #13 of 91 (permalink) Old 03-01-2019, 10:53 PM
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My MPG past 12 months. Top grey plot is fool computer claimed MPG. Bottom is actual miles divided by gallons. Red is a running average.

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post #14 of 91 (permalink) Old 03-02-2019, 04:51 AM
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I have had a careful read of both the Owners, and Service manuals for the FJR. Neither manual explains how the fuel flow is measured, so I assume that there is no dedicated fuel flow measuring device in the system, or they would have mentioned it.

So, how is fuel flow measured? Well, the only useful data that I can think of, which is available to the computer, is "Time" from the bike's clock, and a count of fuel injector pulses, and their pulse duration. If the computer knows how much fuel an injector will pass in X amount of time, at a certain fuel pressure, it can then calculate the total fuel flow.

Now that the computer has calculated fuel flow, you only have to add distance traveled, from the odometer, to calculate fuel consumption.

So, there are a number of places where errors can creep into this system. These being, the condition and calibration of the injectors, fuel pressure, fuel density and temperature, accuracy of the odometer, accuracy of the timing (unlikely), and accuracy of the initial calibration data supplied to the computer. Also of course, the accuracy of the fuel station pump enters into this calculation, as does the accuracy of your refill, are you refilling to exactly the same point every time?

While the condition of other components in the engine, and loading of the bike, will affect fuel consumption, they will not affect the accuracy of the fuel consumption measurement/calculation.
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post #15 of 91 (permalink) Old 03-02-2019, 06:20 AM
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I checked this once when I got my bike, and there was about 10% disparity, like yours.
But I don't really see this as a problem, that's not what the bike is made for...
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post #16 of 91 (permalink) Old 03-02-2019, 06:52 AM
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I think the mpg readout is a bit more simple (but really don’t know) and simply calculated by the fuel tank float position vs actual miles traveled, wouldn’t be precise at all, just a reference number only and something to keep the idiots in Washington happy, maybe.

Add in the usual 2-4 mph speedo error that also drives the odometer and i’d Say that who knows what it really is, and really surprised it’s as close as it is..

I guess i’m Going to have to put the Garmin on the bike and check the speedo & odometer against it, and then it’s a matter of how accurate the Garmin is......last time we had one of these discussions and I actually called Garmin to find out the actual accuracy of their gps I found them to be completely uncommitted to any number, they couldn’t/wouldn’t say anything related to how accurate their gps readout were..

N4,,, you need a hobby!
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Last edited by passx; 03-02-2019 at 06:55 AM.
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post #17 of 91 (permalink) Old 03-02-2019, 07:35 AM
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Quote:
Originally Posted by N4HHE View Post
My MPG past 12 months. Top grey plot is fool computer claimed MPG. Bottom is actual miles divided by gallons. Red is a running average.

WOW..........just WOW.
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post #18 of 91 (permalink) Old 03-02-2019, 07:43 AM
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Originally Posted by Bernie View Post
I have had a careful read of both the Owners, and Service manuals for the FJR. Neither manual explains how the fuel flow is measured, so I assume that there is no dedicated fuel flow measuring device in the system, or they would have mentioned it.

So, how is fuel flow measured? Well, the only useful data that I can think of, which is available to the computer, is "Time" from the bike's clock, and a count of fuel injector pulses, and their pulse duration. If the computer knows how much fuel an injector will pass in X amount of time, at a certain fuel pressure, it can then calculate the total fuel flow.

Now that the computer has calculated fuel flow, you only have to add distance traveled, from the odometer, to calculate fuel consumption.

So, there are a number of places where errors can creep into this system. These being, the condition and calibration of the injectors, fuel pressure, fuel density and temperature, accuracy of the odometer, accuracy of the timing (unlikely), and accuracy of the initial calibration data supplied to the computer. Also of course, the accuracy of the fuel station pump enters into this calculation, as does the accuracy of your refill, are you refilling to exactly the same point every time?

While the condition of other components in the engine, and loading of the bike, will affect fuel consumption, they will not affect the accuracy of the fuel consumption measurement/calculation.
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Originally Posted by passx View Post
I think the mpg readout is a bit more simple (but really don’t know) and simply calculated by the fuel tank float position vs actual miles traveled, wouldn’t be precise at all, just a reference number only and something to keep the idiots in Washington happy, maybe.

Add in the usual 2-4 mph speedo error that also drives the odometer and i’d Say that who knows what it really is, and really surprised it’s as close as it is..

I guess i’m Going to have to put the Garmin on the bike and check the speedo & odometer against it, and then it’s a matter of how accurate the Garmin is......last time we had one of these discussions and I actually called Garmin to find out the actual accuracy of their gps I found them to be completely uncommitted to any number, they couldn’t/wouldn’t say anything related to how accurate their gps readout were..

N4,,, you need a hobby!
My understanding is that with today’s electronically controlled fuel injection systems Bernie is correct. It is based upon the ECU knowing how much fuel is being injected into the engine.

I can’t speak for the credibility of this article, but it does explain it pretty well...
https://mechanics.stackexchange.com/...g-display-work
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post #19 of 91 (permalink) Old 03-02-2019, 07:59 AM
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Quote:
Originally Posted by passx View Post
I guess iím Going to have to put the Garmin on the bike and check the speedo & odometer against it, and then itís a matter of how accurate the Garmin is......last time we had one of these discussions and I actually called Garmin to find out the actual accuracy of their gps I found them to be completely uncommitted to any number, they couldnít/wouldnít say anything related to how accurate their gps readout were..
https://www.gps.gov/systems/gps/performance/accuracy/
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post #20 of 91 (permalink) Old 03-02-2019, 09:31 AM
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As an experiment I filled mine up and set the reset average MPG. Then I checked my mileage over the next ten fill ups, and after ten fill ups, it was pretty close. I don't remember precisely, but I think it was 1 mpg and change.

Each tank, however, varied more. I think the computer has no way to know how much we actually fill it every time, and it varies depending on how the bike's sitting, how level the ground is, etc.

A quart makes a lot more difference on a single tank than it will over several.
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