I'm not convinced that your method is reliable.

Brim-to-brim and caluclate, please.

 

This method used over two or three years and averaged out it`s reliable enough...?

The car`ll do 650 - 700 miles on a tank brim, it`d take another couple of years use to compare methods.

I *think* i used to do brim-to-brims and had odd/lower mileage then, too.

 

Perhaps your driving style/route each time is different enough to cause the discrepancy.

 

The light does not come on at the same time its based on miles left and dependant on short term calculated MPG etc as well as fuel level.

So if you were stuck in stop start traffic when its getting low on fuel light will some on earlier than if you are on a main road cruising

 

^^^so the fuel light is not hooked up to a simple float in the tank, then?
That might make my mpg calculation method a bit inaccurate then..
I suppose i`ll have to go back to filling right up again at £75 a go!!

 

There's loads of things that can change your MPG. Taking a few people one time...not the other. Extra traffic one day. Fuel quality changes. Just feeling a bit tired and floored it a bit more. Car infront holding you back and forcing you to drive slower...at a more economical speed. Or maybe just the temperature. We've had a lot of fluctuations in temperature, 10c one day, 3c the next. Colder air = denser air and the denser the air, the more fuel it takes to push through.

Oh and as discussed before, the fuel level light not always turning on at the same time. Or any mixture of everything.

 

Air temperature will make virtually no difference to fuel consumption with a modern engine. Denser air will be compensated for by the engine management system to keep emissions under control.
Different weather is another story, a slight headwind will have a measurable effect, as will a tail wind. I don't think anyone would get identical fuel consumption figures from consecutive tankfuls in normal driving.

Roger

 

Air temperature will make virtually no difference to fuel consumption with a modern engine. Denser air will be compensated for by the engine management system to keep emissions under control.

I was talking about drag on the car, colder air is more dense, it requires more force to push through. Your engine does compensate for having more oxygen which makes more peak power but that's not something you really need if trying to eke out the last few MPG!

Oh and yes, head/tail wind makes a massive difference too as you said.

 

Air density and temp as well as fuel temp will have a minor impact on fuel economy. If you look at any ecu file maps you will see changes in injected fuel quantity based on these variables to maintain optimum combustion and lamda values.

You will also get different volumes of fuel when you fill with fuel based on the fuel temp/density.

The differences are minor but do have some effect, it one of the reasons aircraft are fuelled by weight because the large volumes of fuel and huge temp variations between the ground and high altitude can have a large impact.

Sent from my SM-T310 using Tapatalk

 

You will also get different volumes of fuel when you fill with fuel based on the fuel temp/density.

The differences are minor but do have some effect, it one of the reasons aircraft are fuelled by weight because the large volumes of fuel and huge temp variations between the ground and high altitude can have a large impact.

Sent from my SM-T310 using Tapatalk

This comes up often on American car forums I subscribe to.
In reality the temperatures of fuel storage tanks in modern filling stations are remarkably stable, certainly not enough to have any discernable effect on fuel volume, liquids have a very low coefficient of expansion when heated anyway.
Aircraft are fuelled by weight because it is sold by the tonne, also the trim of aircraft is a very important factor for flight engineers to account for. The fuel load for a regular passenger aircraft transatlantic flight will be around 130-150 tonnes, it needs to be accounted for and properly distributed.
Roger.

Has anyone done a wind tunnel test using air at different temperatures in otherwise identical conditions?. I doubt there would be any significant differences, but I'm happy to be wrong if someone knows better. Certainly I very much doubt it could be measured by checking odometer readings every tankful.

Roger.

 

..possibly its about driving conditions - my wife's cars shuts down at a standstill and by watching various instant readouts iv'e seen its amazing how much it saves. That car also gives average mph and town driving during a tank of fuel can always make that only 19mph and consumption is high. I've found on my 406 HDI, consumption over a tank can vary from 37-47 mpg - depending on how much town driving i did.

 

It appears that my thermostat is either stuck open or opening far too early and this is affecting the mpg..
Do diesel fuel maps have enrichening during the warm-up phase as per a petrol motor?

It`s not the first time i`ve had `stat issues on this car, i`ve got a new one on order and will monitor warm-up and consumption in due course..

 

Most diesels do not have any major warm up maps, they have higher injection rates when cranking to start.

Once running they run on mostly the standard maps with very slight increases in fueling due to extended injection timings due to slightly increased combustion times based fuel and coolant temps etc

 

Due to previous concerns on this car i have a digital temp sensor fitted to the radiator top hose to monitor coolant temp. The display is on the dash where i can see it.
If my memory is correct this was the observations i saw: the digi gauge used to show no change in temp - it`d be at ambient - for approx the first 2- 3 miles of driving from cold then would show a definite and sudden increase as the thermostat opened, which is what you would expect. The dash coolant temp gauge would then settle on ~65 degrees (rather low...) and the digi gauge on the rad hose about ~55 - lower than actual coolant temp because the digi gauge is measuring the rubber hose surface area which is probably quite a good thermal insulator to the coolant within.
Anyway, the reason i suspected the `stat this time was that the digi gauge on the rad hose was showing a gradual increase in temp from the off - as soon as you drove from cold the coolant in the rad hose was increasing, leading me to suspect the `stat was stuck open. This would also explain the low coolant temp displayed on the dash gauge.

I put a new thermostat in yesterday (yet another one - the fourth this car has had in 9 years...) and took it for a test drive.
I was dissapointed to see the same initial increase in rad top hose temperature straight from cold displaying the same behaviour as the previous `stat....
The rad hose got to about 25 degrees after 2-3 miles then levelled off for a while, then dipped slightly before rising again to a steady 70 degrees. The dash coolant temp gauge settled on 80 degrees. From this it could be seen that the `stat was opening and behaving correctly but why the rad top hose is getting warm before the `stat fully opens is still a slight mystery - unless my memory of how it used to behave is wrong..
I`ve had this car nearly 9 years and i`ve never been entirely satisfied that the coolant temp and thermostat were behaving properly (previous new thermostat fitments didn`t seem to make much difference) and i`ve never seen the dash gauge sit at 80 degrees which is where i`d expect it to be.

So, the new `stat has raised the running temp about 15 degrees, hopefully this`ll be the reason why my fuel consumption dipped by 25%.
I`ll update after a few tankfulls.