Spurious range estimates

I never pay much attention to the range estimates produced by the computer in my Mini Cooper S. Today however, after eking 350 miles out of a
tankful, my most ever, I was puzzled/amused to see that the computer estimated that I could get 459 miles out of the next tankful.
This will never happen unless I can find a 459 mile downhill stretch of road.
Is the a BMW “feature”? My other half’s car computers have never been so wildly inaccurate (VAG group vehicles).
Tim
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I've found both of mine extremely accurate. Better than just the fuel gauge. But they work out the remaining range based on the consumption at the time. Hence with not much fuel in the tank, the distance till empty will increase as the engine warms up after a cold start.
So because it says 459 at the moment, it will very likely update that as you drive, and if your more usual figure is rather less quickly arrive at something like that.
Because it can't possibly know how the car is going to be driven when you fill up. Just guess.
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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Dave Plowman (News) wrote:

Some years ago on a petrol Avensis the trip computer was consistent in its estimates. The average from Jan-Aug 2005 was about 41.8 mpg.
However, it indicated a zero range when there was still about 5 litres of fuel in the tank. So on one occasion I took a can of fuel with me and ran the car to empty. It ran about 50 miles from first indicating zero range before it finally stopped. Meanwhile the fuel guage dropped slowly from the "Empty" mark down to the end stop that the pointer rests on.
The conventional wisdom is that the range estimate is always very pessimistic, otherwise drivers would run out of fuel far too frequently.
--
Graham J


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On 10-Apr-18 10:23 PM, Graham J wrote:

Why run to empty? It doesn't do any fuel system any good to pump froth.
For petrol injection the high pressure pump in the tank is cooled by the fuel, running on air will shorten it's life. Direct injection very high pressure pumps are poorly lubricated by petrol and air won't go down well.
For diesel you may have to prime the system. Even if it self bleeds you will have to abuse the starter and crank it for way longer than is really good for it (unless it's stop/start that has a higher rated starter motor). With some the priming takes so long you risk a flat battery.
It's far easier to look up the tank capacity in the owners manual. When the warning light comes on brim the tank. Reserve = capacity - fuel to fill from warning lamp.
For mpg, fill tank, zero the trip, drive until near or to warning and then brim it again. Distance*4.54/liters is the mpg you achieved on that run.
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My manual states 43L. Yesterday I put 44L in. I have managed 45L before. Clearly the manual information can’t be trusted.
Tim
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On 11/04/2018 11:23, Tim+ wrote:

or the pumps can't be trusted :)
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MrCheerful wrote:
[snip]

That may be a serious accusation that you could take to Trading Standards ...
see:
https://www.resolver.co.uk/consumer-rights/petrol-stations-pump-inaccurate-complaints
--
Graham J




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Much more likely that the manual is wrong though... ;-)
Tim
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It does depend how accurate the fuel level sensor is. I had one BMW with twin units in the tank, and did run that totally out of petrol intentionally after first getting it and it showed minus 15 miles to go. Close enough for me. I generally fill up when the reading drops to 30 miles or so. With the warning light on. My days of attempting to run on air are thankfully behind me. ;-)
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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On 11/04/2018 00:34, Dave Plowman (News) wrote:

Thirty years ago I always had a gallon in a can in the boot, now there is just no need. Also had water, brake fluid, oil, fan belt and a spare wheel. Now: mobile phone and RAC card.
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Oddly, with so many filling stations closing - another I sometimes used just has - there may be more need to carry that can of petrol than a few years ago.
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*They told me I had type-A blood, but it was a Type-O.*

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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On 11/04/2018 10:28, Dave Plowman (News) wrote:

back then there very few petrol stations open at night, but there were loads of little tiny stations with just a couple of pumps. near me there are 7 big places within a two mile radius of home and all but one are 24 hour and all but one have a mini supermarket too.
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On 11/04/2018 00:34, Dave Plowman (News) wrote:

motorway journey. It must re-generated somehow - at least according to the fuel computer.
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The estimate on range is based on the way it is driven, weighted towards the most recent mileage. if you drive with an extremely light foot for the last few miles before a refill, or your route to the refill is downhill for a few miles then the range estimate on refill will in my experience always be higher. Drive as if you stole it in those past few miles and the estimated range can be much lower.
I think we've had just over 800km (500 miles) estimated range on our Cooper (R56 petrol non S) we keep in Italy which would be more than 60mpg if it were true.
The most it's actually ever done on a tank (40 litre official tank capacity plus at least a couple of litres, possibly a bit more in the headspace and neck) on a very long run with two up, a full boot and seats full to just below window level) was around 700 ish km driving at around 100kmh which worked out at around 53mpg when brimming the tank.
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Well that’s certainly how it seems but it seems an idiotic way to do it. There are so many more data points that could be used that would give a vastly more accurate estimate.
Tim
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Such as? How can you do an accurate estimate without knowing how the car will be driven during the next tankful? 150 mph on an autobahn, or a gentle cruise in the country at 50 mph?
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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Imagine it’s an exam question.
Your car has a 10 gallon tank. You drive 345 miles at 35mpg followed by 5 miles at 45 mpg. What it the likely range for the next tankful? Are you really gonna say 450 miles?
Do you really think basing your answer on the last 5 miles consumption is a sensible way to calculate it when you know what the consumption was for the first 345 miles? No, it’s clearly an idiotic way to do it and I don’t know what BMW we’re thinking of when they designed it. I’ve never had a car that generates such spectacularly spurious results before.
Tim
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On 16/04/2018 08:38, Tim+ wrote:

I would answer "450 miles if I continue to drive as I did the last 5 miles. 350 miles if I drive as I did the previous 345 miles. 351 miles if I drive at the average MPG of the previous 350. Insufficient data to estimate likelihood of those or the infinite number of other possible figures."

And ISTM an average over _recent_ driving is the sensible thing to do - bearing in mind for most users the issue is not "how far can I go on a tankful?" but "how far before I need to refuel?"
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Robin
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On 16/04/2018 09:38, Robin wrote:

:-) . . . I think that's about right. The computer on my 2007 Audi does an as-you-go estimate of tank range, seemingly based on the past few miles.
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Of course It has to. Little point in the thing saying you've only got a 150 miles range because the car was thrashed on the last tankful. When you're actually going to get 300 out of this one when cruising.
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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