[Jamesi30]

I did an economy run today from Brisbane to Gympie and achieved 5 litres per 100 km keeping to around 90km/hr. This figure came from the trip computer.

And I got the number 1 ranking and a perfect economy score.


The Autolink app however says I did 6.5 litres per 100km. ( calculated from its figures for distance travelled and fuel used )

This doesn’t agree with the trip computer or my calculations when I filled up.

Adding up all the figures for fuel consumption from the app since my last fill showed it recorded 28 litres of usage when in fact I only used approx 23.

I think the app is in the very early stage of development and quite buggy.

Has anyone else noticed this?

[Dazzler]

Hi James,

It won't even work consistently with my Samsung S8. It seemed to like my LG G4 better, but it could be the software itself because it has had a couple of updates since I got my car 16 months ago.

I have a service booked 2moro so I might see if they can get it working.

You would think it would be as accurate as the trip PC which is within a couple of points usually. 

A few have reported bugs on here or, like me, pretty much given up on it. 

[AlanHo]

I believe in the principle that the only way to calculate the accurate economy figure is to know exactly the distance travelled on a tank of fuel and the amount of fuel used.

I measure the amount of fuel used by noting the odometer reading the first time I refuel - and I brim the tank. By brim I mean fill the tank until you can see fuel in the filler neck and it is stationery. Refilling to the first, second or third click is inconsistent, especially if you use different pumps.

The next time I refuel, I brim the tank in the same way - I now exactly how much fuel was added and by deducting the previous odometer reading from this one, the distance travelled. I do this for every tank and record the figures in a spreadsheet every time.

The trip recorder in my car is inconsistent and since I bought it has showed a figure anything between 3% pessimistic and 9% exaggerated.

Before members post a warning  - I will.

Brimming the tank, if you are not driving a reasonable distance immediately afterwards, carries the possibility that the fuel will expand and spill onto the road. Diesel on the road is a hazard, especially for bikers.
Hence I always refuel part way into a journey and not just before getting to a destination.

I have gone to the trouble of checking how accurate my odometer is. In the UK there are distance boards showing the exact number of Km from the start of the motorway. It is easy to compare your odometer with these markers over a long journey of say 200 km. The longer the better. The odometer in my current car is 1.05% pessimistic - in other words when I have travelled 100 km according to the odometer - the actual distance is 105 km

The odometers in my previous cars have all been pessimistic - for example :-
2016 Venga - 2.2%
2013 i30 - 1.4%
2012 i30 1.8%

I have kept a spreadsheet of my fuel consumption for every car I have owned recently which includes an Audi A3 diesel, three i30 diesels, two Kia Venga petrol, and now a Kia Niro petrol hybrid. All of them have had pessimistic odometers and inaccurate trip meters

Here is the Niro spreadsheet

[Newyi30]

What's 'Fuel Efficienty'?
Maybe that's your problem... 

[Jamesi30]

Hi Alan,

Thanks for your reply.

I have noticed that although my speedometer is very optimistic by about 5% the odometer figures seem consistent when compared to gps recordings of my trips.

I agree with your point that filling your tank and relying on the pump to fill to the same level can lead to errors. My experience is that different pumps can stop where others would have put in 5litres more.

You would think by the way that they detect a full tank the level would be the same. The temperature of the fuel coming out of the pump would also change its volume. I wonder if the pumps compensate for temperature when measuring the quantity of fuel delivered.

I shall try to take out the possibility of errors in filling the tank by filling at a pump prior to a trip in the morning and using the same pump and time the next day. This should eliminate temperature and quantity delivered variations.

I would have thought the trip computer the most accurate measurement because it is data coming from the engines computer. It knows exactly how much fuel it has injected.

I am just wondering now if the app has some erroneous conversions between units of measurement. I know that one commercial air crash was simply because of the plane running out of fuel because of errors of understanding between the ground crew and the pilots in the units of measurement used putting fuel into the plane.

Cheers

Jamesi30

[Cookie Thumper]

Hi James,

It won't even work consistently with my Samsung S8. It seemed to like my LG G4 better, but it could be the software itself because it has had a couple of updates since I got my car 16 months ago.

I have a service booked 2moro so I might see if they can get it working.

You would think it would be as accurate as the trip PC which is within a couple of points usually. 

A few have reported bugs on here or, like me, pretty much given up on it. 

Yeah it does not like my S8 either, I mentioned this when I got the car serviced and just a blank stare.

[AlanHo]

The temperature of the fuel coming out of the pump would also change its volume. I wonder if the pumps compensate for temperature when measuring the quantity of fuel delivered.

I would have thought the trip computer the most accurate measurement because it is data coming from the engines computer. It knows exactly how much fuel it has injected.

Jamesi30

@Jamesi30 - It is a long time since I was involved with fuel measurement - but if my memory is correct  -  the pumps at service stations in the UK are positive displacement and can deliver fuel within an accuracy of about plus or minus .001%. Which means if you put 40 litres into a tank you get a minimum of 39.96 litres and a maximum of 40.04 litres - a variation of about about 40 ml or 8 teaspoons.

However, whilst the pump will measure flow very accurately, the amount you are paying for will be less accurate because fuel varies in density according to temperature - it expands when warmed and contracts when cooled. The pumps in the UK are calibrated at a temperature of 15*C so if the fuel is warmer than this you get less for your money. A typical value of the coefficient of thermal expansion for diesel fuel is 0.00083 per degree Celsius.

In the UK, fuel tanks are underground. The average underground temperature at a depth of 2 metres is about 12.5*C throughout the year -  hence the fuel delivered to the vehicle is more likely to be below 15*C rather than above it.

I recall a few years back the AA did a survey of UK service stations and checked the accuracy of the pumps. They were surprised to find that more pumps were delivering more fuel than paid for - than those delivering less fuel. They did find some howlers though - I recall one service station was about 4% out.

If the temperature was say 12*C you would get an extra 0.1 litres when you put 40 litres into the tank.

Hence actual temperature variations - in the UK - are no big deal.

Now turning to the question of trip computer accuracy depending on the ECU's calculation of the fuel delivered. I am not aware that a car has a flow meter in the fuel line and assume that it is calculated by the amount of fuel the ECU asks of the injectors which are opening and closing at - I guess - between 6 and 50 times per second.
However - in a car, fuel temperature variations are much greater than in a service station and are likely to play a part in the accuracy of the fuel delivered compared with the amount the ECU asked for. I don't know whether temperature sensors are in the fuel line to allow the ECU to compensate. If not, the trip computer will be fooled.

Which leads me back to my assertion that the only figure you can really trust is a manual calculation of the actual fuel used over the distance travelled. Even then it will be slightly effected by the temperature of the fuel you purchased.

I have read on forums that some people refuel early in the morning when the fuel is likely to be at a lower temperature than in the afternoon. I have done the maths - it just aint worth the fuss for a few teaspoons of extra fuel.

[Jamesi30]

I am using it on an iPhone X. Previous releases have been stable but more recent ones crash a lot or do not connect properly to the Bluetooth module.

It is beta release technology I think which is why it has limited release to only certain models in Australia when I purchased the car. Essentially I am a guinea pig 😁

[Purplehazeffc]

Auto Link is very hit & miss for me. Using a Huawei Mate 10.
The "live" part of that app doesn't work at all. So the rpm etc shows 0 rpm.
But if I leave it on & just drive. When I get to say work. Which is 60ks away.
Turn the car off. The app will show the ks done & fuel used..

My speedo is 5 kph faster than shown. Doesn't matter if I'm going 50kph or 100kph.
Not sure about the oddo though. I may have to check that.