Author Topic: Accuracy of Range Estimate ?  (Read 3601 times)

ColinB

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Accuracy of Range Estimate ?
« on: September 20, 2016, 06:34:34 PM »
On a longish trip today I was interested in whether I'd need to refuel before getting home, so I was looking at the computer's range estimate. I know this is only approximate, but even so I was really surprised at the variability of the numbers. At one point just after fuelling the estimate was over 500 miles, then it dropped suddenly to 438 miles, and a bit later still it was back up to 495 miles, before dropping back to around 460. In between those step-changes there seemed no pattern to how it changed, sometimes it didn't change at all for several miles but sometimes it gradually reduced as you might expect it to mile-by-mile. All in the space of around 30-40 miles actual distance covered. I expected this to be a fairly simple calculation for the computer involving the fuel quantity in the tank and the average MPG at the time, but it's clearly more complex than that, it was almost as if the range was recalculated from scratch periodically, with "dead reckoning" between those times. I can understand the 500 figure being incorrect straight after fill-up with the MPG reset and the fuel gauge being misled by the fuel being brimmed to the top of the filler neck; you'd expect it to change significantly as the MPG sorted itself out and the fuel gauge started to register the actual level in the tank, but I don't understand why it then increased back up to nearly the start value. I guess there're so many possible sources of error in both the measurement of the tank contents and the MPG calculation that you really can't expect anything reliable out of it.

culzean

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Re: Accuracy of Range Estimate ?
« Reply #1 on: September 20, 2016, 06:51:56 PM »
It probably keeps calculating miles to go based on present rate fuel is being used. So if you have a period of high usage it will lower estimated remaining miles. And as with average mpg as it gets a few miles on the trip it becomes less sensitive to fluctuations in fuel usage (up or down hill etc) as it is averaging rate of use over a greater number of miles.
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mikebore

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Re: Accuracy of Range Estimate ?
« Reply #2 on: September 20, 2016, 07:00:41 PM »
I notice that as you approach empty the estimate of miles remaining reduces more rapidly than the miles covered.

I calculated that I should get off the motorway OK based on miles left with two bars showing. But it went to zero bars  and zero miles well before that and we had to turn off motorway to fill up.

The Mk2 was the same. I had hoped with the less conservative approach on the Mk3 this wouldn't be the case.

Skyrider

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Re: Accuracy of Range Estimate ?
« Reply #3 on: September 20, 2016, 07:48:14 PM »
I don't do fuel gauge stress, I take 1/4 full as a hint that the car needs fuel.

culzean

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Re: Accuracy of Range Estimate ?
« Reply #4 on: September 20, 2016, 08:23:36 PM »
I don't do fuel gauge stress, I take 1/4 full as a hint that the car needs fuel.

+1 me too, 'because life's too short to worry about little stuff'
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andruec

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Re: Accuracy of Range Estimate ?
« Reply #5 on: September 20, 2016, 08:58:41 PM »
I used to fill up when the gauge reached half way. But for some reason once the cars I owned switched to horizontal gauges I was happy to let them drop a lot further. With my Mk2 I was happy to wait until the amber light came on but I'm more leary of that with the Mk3.

Either way generally I fill up Sunday evening if I think I have less than 150 miles left in the tank as that's my weekly commute. At the moment that's when the mileage display is showing 300+ miles driven

mikebore

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Re: Accuracy of Range Estimate ?
« Reply #6 on: September 20, 2016, 09:07:48 PM »
I don't do fuel gauge stress, I take 1/4 full as a hint that the car needs fuel.

No I don't normally, but this was soon after getting the car and I wanted to see what the miles remaining number was when the low fuel warning first showed.....since it was so pessimistic on the Mk2. See this thread.

Normally I fill up on 2 or 3 bars.

KentEx

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Re: Accuracy of Range Estimate ?
« Reply #7 on: September 21, 2016, 06:31:11 AM »
Its the same on mine. I'm wondering could it be that when you refuel after previously having a long economical run, the initial figure is high, then with the new tank of fuel you drive with more stop/start small runs so the figure drops?
I've also noticed that once the range estimate gets below 80 miles it falls away very rapidly, I've got no theories on that one.
Whatever the reasons at the end of the day the figure is an estimate and I suppose should be treated as such.

John Ratsey

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Re: Accuracy of Range Estimate ?
« Reply #8 on: September 21, 2016, 08:22:47 AM »
In my experience the Mk 3 gauge is noticeably less pessimistic than on the Mk 2 and it seems to be more linear overall. I've twice gone down to 1 bar remaining on the gauge (and < 10 miles remaining on the computer) and each time managed to put in just under 37 litres which means at least 3 litres remaining plus however much fits in the filler pipe.

One benefit of the less conservative gauge / low fuel warning system is the it effectively gives the car more range between refills. When not near empty, the computed miles remaining is influenced by recent driving conditions. Once near empty the miles remaining increases faster than the incremental miles travelled - it might well be the same factor as on the Mk 2 but only starts nearer to empty. The indicated range after a refill appears to be based on overall average mpg (actual, not the optimistic mpg indicated by the vehicle).

mikebore

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Re: Accuracy of Range Estimate ?
« Reply #9 on: September 21, 2016, 08:35:29 AM »
One benefit of the less conservative gauge / low fuel warning system is the it effectively gives the car more range between refills.

Yes indeed. On the Mk2 even though I knew full well that there was close to 100 miles left after the warning light came on, I would fill up because I don't like driving with warning lights on.

As you say, the better mpg of the Mk3, combined with the less conservative warning light, effectively gives the Mk3 much bigger range than the Mk2 between fills.

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