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Old 13th November 2011, 08:52   #1
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Default How accurate is the distance to empty

Hi all,

For quite some time now I have this question in my mind.

The modern day technology has got us a new calculation meter that shows the distance to zero or empty in the car. I have noticed this in Tata Manza and now the new Fabia.
My question is how accurate and how dependable is the distance to zero. Last week I filled in 15 litres of petrol in the new Fabia and as soon as the tank was filled it showed me 220 Kms to zero. I drove around 70 kms in the city and suddenly this meter shows 100 kms to zero.
I do not know how dependable is this ?

Experts advise please.
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Old 13th November 2011, 12:14   #2
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Default Re: How accurate is the distance to empty

I am no expert but i am quiet sure in the few cars that family/friends own this has give them massive surprise occasionally. The distance to empty is only indicative and folks have a few times got stranded midway blindly depending on this information.

Also this has been an experience across multiple cars across different manufacturers. So on the safer side its better to believe that when the fuel indicator needle hits the red mark (assume approximately there is 10L or fuel left, although in reality its a little bit more).
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Old 13th November 2011, 13:51   #3
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Default Re: How accurate is the distance to empty

I have found the DTE too optimistic at the time of tanking up, but it settles down to more estimate once on the move for some time. The DTE in my Elantra had more variation than any other cars i have owned. The display goes blank when 50kms range is left for most cars i have seen, this is mentioned the car's manual for all cars.

On the other side, i find the trip computers in most cars pretty close. For all the times i have cross checked in Punto and Laura, the trip computer's calculation was not much far off. I do not mind the trip computer as far as it within +/-1kpl of my manual calculation.
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Old 13th November 2011, 14:04   #4
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Default Re: How accurate is the distance to empty

I do not have these features in my car. But I do a mental distance to empty based on the fuel gauge bar display. And here in crops up the problem. In my meter under normal driving conditions the distance covered for each bar is
10 - full tank
9 - 60
8 - 90
7 - 120
6 - 150
5 - 180
4 - 210
3 - 240
2 - 270
1 - 300
blink - 320
That is 60 for the first bar and 30 (but varies from 20 to 40+ at times) for each successive bar. But I have got as much as 550 km and as low as 280 km on a tank full. Thus if I go by normal case, when the bar is at 3, I should have 80km left, but in reality, it can be between 50 and 150km, depending on how I drive there after. The computer does basically what I do (may be more scientifically), but neither of us can predict how far the remaining fuel will last, with any accuracy. All that the computer can do is to tell you that at the current rate of consumption you will go so far, as you have so much fuel left.

Thus if driving in city, I fill up once the bar starts blinking, and on highway I assume that I have 100km left when on Bar 3, and start looking for a reliable petrol pump (I may get 150 or more, but keep the 50km or so as a contingency). Usually I try to do my home work and identify reliable pumps on my route and fill accordingly, which with modern Maps and forums like TBHP, is quite easy these days.
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Old 13th November 2011, 14:23   #5
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Default Re: How accurate is the distance to empty

In my experience with Skoda Yeti, the distance to empty indication appears to be based on the average fuel consumption over last several kilometers.

With near full fuel tank, on highways it tends to get over-optimistic showing me a range of 1000+ km.

On the occasion when I tanked up with a range showing barely 20km I was able to take in just 50 litres to full tank, whereas the capacity is 60 litres. I therefore guess that in this case the estimates are over optimistic with plenty of fuel and get progressively over-pessimistic near depletion.

The same appears to be true for Honda Jazz as well.
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Old 13th November 2011, 14:36   #6
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Default Re: How accurate is the distance to empty

Quote:
On the occasion when I tanked up with a range showing barely 20km I was able to take in just 50 litres to full tank, whereas the capacity is 60 litres. I therefore guess that in this case the estimates are over optimistic with plenty of fuel and get progressively over-pessimistic near depletion.
Well, this is true and my only fear is assuming that the car DTE shows 50 Kms and if i end up driving 5 or 10 Kms and then if it shuts off forcing me to push the vehicle.

Last edited by Rudra Sen : 14th November 2011 at 11:12. Reason: fixing quote
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Old 13th November 2011, 15:16   #7
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Default Re: How accurate is the distance to empty

First of all, the Distance to empty is an indicator based on the average fuel consumption over the last tank ful. As Sushil rightly said, if the car has spent its last tank full in the city, the DTE will show much lesser than if the car has done a highway trip. For example, my Verna CRDI has a 45 ltr fuel tank and gives approx 13-14 kmpl in the city. On a full tank, it shows 580km to empty. However, on the highway it gives about 20kmpl. The last time we drove back from Vizag and refuelled the tank, it was showing 880 kmpl!

Secondly, a DTE typically tends to be more accurate at brimful than if you fill part of the tank. I'm not aware why the latter happens, but maybe the computer sensor cannot gauge the quantity of fuel correctly!
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Old 13th November 2011, 15:53   #8
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Default Re: How accurate is the distance to empty

If you look at the FE calculator, you'll observe that it changes every second; when you're not acceleration, it maxes out and it decreases when you accelerate.

Every second of driving, the FE changes; the DTE depends on the amount of fuel in the car and the FE.

So, if the FE itself is changing every second, so will the DTE. It is only when you're idling or driving steadily that you'll get a realistic DTE figure.
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Old 13th November 2011, 16:32   #9
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Default Re: How accurate is the distance to empty

This is what used to happen with my Honda city (current generation)
When you fill the tank and reset the trip meter, the FE displayed is too good and even the distance to empty (range) also stays without much changes. After driving for the forest 50 kms, it slides and and stabilizes. But during the last 75 kms the indicated FE really drops and the 'range' also starts dipping very quickly. 75 kms of 'distance to empty' will turn to zero in less then 25 kms!
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Old 13th November 2011, 18:45   #10
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Default Re: How accurate is the distance to empty

The distance to empty is indeed based on the average fuel consumption. However a major point to note is when fueling, you should ensure all the air bubbles in the tank/pipes are got rid of (by shaking the vehicles). This would ensure a more accurate distance to empty readying following the consumption.
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Old 13th November 2011, 20:08   #11
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Default Re: How accurate is the distance to empty

The Yeti manual says that range to go estimates are based on the average fuel consumption calculated in the past 50 km. I guess each manufacturer uses its own algorithm to calculate the range, and they don't necessarily reveal the logic to enduser creating a confusion.

I feel that the manuals are often sketchy and ambiguous on instrumentation. The manufacturers should provide enough information about the logic (algorithm) - together with probable errors - used for various instruments (particularly with regards to instruments like range to go) so that the user can informed decision and not end up with surprises.
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Old 13th November 2011, 22:06   #12
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Default Re: How accurate is the distance to empty

It's a good *indicator* but you can't treat it that seriously. In my Vento AT, with full tank it shows around 420 km (I can easily get 550 on my normal routine). Towards the end of the cycle it tends to get even more pessimmistic- I have gone as low as 10 km DTE and discover I still have around 5 litres (estimated) in the tank. No complaints though- that is how it should be.
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Old 14th November 2011, 00:44   #13
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Default Re: How accurate is the distance to empty

Quote:
Originally Posted by libranof1987 View Post
So, if the FE itself is changing every second, so will the DTE.
Not really. In most cars, it averages out the FE for the last 100 + kms or so.

AFAIK, regarding how the DTE works:

Most modern ECU's can give out an instantaneous FE figure. Now, what the DTE does is to average this FE attained along the last few hundred kilometers and compare it with the fuel remaining in the tank, and give out the math.

About how reliable the DTE is:

DTE is a variable and depends upon the parameters.
  1. Average FE.
  2. Fuel remaining
1. Average FE: There are many unforeseen variables which affect the instantaneous/average FE, like, fuel quality, vehicle condition (which includes the fuel supply, air supply, tyre air pressure, compression etc..etc.. ) altitude, driver attitude, number of passengers, load carried et al.

2. Fuel Remaining: AFAIK, the fuel remaining is just a perceived value and is in no means accurate. Even though, years of development and newer technologies have made it close to accurate, in todays cars, we can expect <5% of error.


So, considering both the factors are variables, one can only expect a rounded up figure for the DTE.

I think OP gets my point.
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Old 14th November 2011, 08:42   #14
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Default Re: How accurate is the distance to empty

I do not have a DTE in my Ford ikon but on my bosses FIGO Petrol. The DTE shows 0km but he still drives around for about 30 km before refueling.

While this may look like inaccuracy , I think this was intentionally done to induce the owner to refuel. Petrol cools the intank pump etc. He claims that he can drive almost 50 kms after it shows 0km on DTE.

OT - In my ford ikon , My brain does the DTE as I follow tank to tank method. And reserve shows at about 40kms. Maybe the reserve on regular is 0 km for DTEs.
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Old 14th November 2011, 08:53   #15
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Default Re: How accurate is the distance to empty

Quote:
Originally Posted by dhanushs View Post
DTE is a variable and depends upon the parameters.
  1. Average FE.
  2. Fuel remaining
Very neatly put. It not possible to provide a precise figure (even with +/- ??)

It depends on the way the vehicle is driven and even if they match the pattern, it cant be perfect. Because, not all drivers drive it same way.

So, it has to be used with some "experience" and caution.
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