Author Topic: MPG for those interested.  (Read 31693 times)

ColinB

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Re: MPG for those interested.
« Reply #30 on: September 29, 2016, 08:43:15 AM »
There seems to be a view on here that the computer's MPG read-out is less accurate than the manual fill-to-fill method. Can anyone suggest why that might be ? The computer must have access to accurate data about fuel flow rates (if it didn't, the engine probably wouldn't run well) and distance travelled (for legal reasons), so surely the MPG calculation ought to be fairly accurate ... so what's going wrong ? What are the likely sources of error in the instrumentation or the calculation that can result in an error of around 10% ? And are there any legal requirements (maybe in the Construction & Use Rules ?) about the accuracy of the calculation ? Just wondering.

andruec

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Re: MPG for those interested.
« Reply #31 on: September 29, 2016, 09:04:10 AM »
There seems to be a view on here that the computer's MPG read-out is less accurate than the manual fill-to-fill method. Can anyone suggest why that might be ?
They always are.

https://www.google.co.uk/search?q=why+car+mpg+display+wrong&rlz=1C1CHBF_en-GBGB704GB704&oq=why+car+mpg+display+wrong&aqs=chrome..69i57.5534j0j7&sourceid=chrome&ie=UTF-8#q=car+mpg+display+wrong

 :)

Or for a simpler explanation:

https://www.youtube.com/watch?v=cOeKidp-iWo

 :D

Even fill-to-fill isn't without issues. Using a different pump, different temperature. I also question how quickly the figure drops when you first use the vehicle from cold. Like last weekend. I was down to the last few litres. Car had done normal trips for a couple of weeks. You'd think the display had pretty much settled down to an average.

Yet within a minute of me setting off from work it had dropped from a tad under 57 mpg to 51 mpg. How can the mpg after driving half a mile affect the average mpg calculated over 400 miles by that amount?
« Last Edit: September 29, 2016, 09:09:30 AM by andruec »

ColinB

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Re: MPG for those interested.
« Reply #32 on: September 29, 2016, 09:43:05 AM »
There seems to be a view on here that the computer's MPG read-out is less accurate than the manual fill-to-fill method. Can anyone suggest why that might be ?
They always are.
There's usually a reason (or reasons) for this kind of thing so I was trying to see whether anyone could explain. Some useful stuff in that blog, although it seems to me that some of those factors affect the manual method as well (eg thermal expansion of the tank), so we probably shouldn't consider either method as 100% accurate.

ColinS

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Re: MPG for those interested.
« Reply #33 on: September 29, 2016, 01:39:54 PM »
The simple fact is that it is illegal for the speedometer to read low, therefore all manufacturers calibrate them to read high.  Everything is linked to that, including the odometer, so the MPG computer calculation is artificially low.

Petrol pumps are covered by "weights and measures" in the UK and are therefore accurate.

Believe the pump not the car.

mikebore

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Re: MPG for those interested.
« Reply #34 on: September 29, 2016, 04:46:51 PM »
I understand about legal requirement for speedometer not to read low, but surely that is not true for the odometer, which is what the car computer uses for the calculated mpg.

If the odometer was also wrong it would affect the manual tank fill method too.

andruec

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Re: MPG for those interested.
« Reply #35 on: September 29, 2016, 04:50:01 PM »
If the odometer was also wrong it would affect the manual tank fill method too.
Tyre diameters reduce as they wear so any measurement derived from wheel rotation is inevitably going to be inaccurate ;)

MPG could be accurate with respect to the odometer but never accurate with respect to miles actually driven.

mikebore

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Re: MPG for those interested.
« Reply #36 on: September 29, 2016, 05:56:38 PM »
I don't believe my odometer is 10% wrong to true miles, which is what is implied by suggestion that the discrepancy is due to the legal requirement for displayed mph.

Skyrider

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Re: MPG for those interested.
« Reply #37 on: September 29, 2016, 07:02:43 PM »
I really don't care about fuel consumption to the nearest egg cup full to the mile. My Jazz does about 100 miles per 1/4 tank of fuel around town, more on the motorway, a bit more on the first quarter tank due to the long filler pipe full being added. What is the big deal (obsession) about absolute accuracy? it will never be achieved on the road.
« Last Edit: September 29, 2016, 07:04:43 PM by Deeps »

VicW

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Re: MPG for those interested.
« Reply #38 on: September 29, 2016, 07:22:30 PM »
Deeps +1

Vic.

Skyrider

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Re: MPG for those interested.
« Reply #39 on: September 29, 2016, 08:05:26 PM »

culzean

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Re: MPG for those interested.
« Reply #40 on: September 29, 2016, 08:36:56 PM »
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

Downsizer

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Re: MPG for those interested.
« Reply #41 on: September 29, 2016, 10:25:23 PM »
I've always monitored actual (pump-based) consumption because a significant change could suggest something wrong with the car.  I only reported the trip computer variance out of interest.  I think the difference originates in the way the fuel use is measured rather than any odometer error.

KentEx

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Re: MPG for those interested.
« Reply #42 on: September 30, 2016, 06:59:24 AM »
Deeps - I'm with you 100%
We all want I suppose the best economy, but to insist on pin point accuracy is just a complete unnecessary mind bender. The instruments are for guidance and if they are within +/- 10%, that's good enough for me. Too many other things in life to get us worked up about and stressed out. Where we should be concerned is where there is a dramatic change.
Lets get on with driving!

ColinS

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Re: MPG for those interested.
« Reply #43 on: September 30, 2016, 07:46:57 AM »
I don't believe my odometer is 10% wrong to true miles, which is what is implied by suggestion that the discrepancy is due to the legal requirement for displayed mph.
Thinking about it, logically you must be correct on this.  Like Deeps, I'm not bothered by it, but it definitely reads higher on the computer than actual MPG.  My last CRV was exactly the same.

ColinB

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Re: MPG for those interested.
« Reply #44 on: September 30, 2016, 08:35:21 AM »
There's clearly a group of folks who aren't particularly concerned about MPG and how it's calculated and I respect that. But the thread title is "MPG for those interested" and a number of other people who posted earlier clearly are interested, so perhaps I could respectfully ask those who aren't interested to allow that discussion to continue please ?

For me, I'm just interested in whether anyone can put up any plausible theories as to why two methods of measuring the same thing produce significantly different results. Doesn't mean I'm anal about the last fraction of an MPG, it's just an objective academic interest (call it an enquiring mind). I don't buy the "That's just the way it is" theory, there's got to be some underlying reasons. And some of the reasons suggested so far may also affect the supposedly accurate manual fill method ... which my sad mind finds quite interesting.

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