Diesel consumption query

[Ben Nevis]

I've also always known that speedos slightly over read. Does this mean that the odometer isn't accurate by the same percentage? And following on does it mean the only truly accurate mpg is from a full to full fill up and distance reading from a GPS device such as a sat nav?

It's an interesting question. There are so many variables including different calibration of diesel dispensing pumps at every filling station, differing tyre types and wear give rolling radii, innacuracy of GPS distance on twisty roads and up/down hills and mountains???

 

[Trebor]

It's an interesting question. There are so many variables including different calibration of diesel dispensing pumps at every filling station, differing tyre types and wear give rolling radii, innacuracy of GPS distance on twisty roads and up/down hills and mountains???

Not to mention temperature.

 

[Trebor]

I use Sprintmonitor to record my mpg. It's based in Germany so gives you the option to compare your consumption with lots of similar engine/gearbox combinations without needing to be a California (eg Multivans, Caravelles).

Please can someone cleverer than me answer this:

I'd always assumed that if I used full to full tank filling measurements and distance travelled from the odometer I'd get completely accurate mpg. I've also always known that speedos slightly over read. Does this mean that the odometer isn't accurate by the same percentage? And following on does it mean the only truly accurate mpg is from a full to full fill up and distance reading from a GPS device such as a sat nav?

I always thought speedos over read by 10 % on new tyres .

 

[Amarillo]

I calibrate my bicycle computer by doing a roll measurement, measure in mm, one complete revolution of the front wheel and enter that measurement into the computer. It makes a significant difference if I'm on or off the bike. Tyre wear and tyre pressure also have a noticeable effect. If speedometers and consumption figures also rely on wheel revolutions, I can understand why they might not be accurate.


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[Kmann]

What App is that?

There are several out there. Mine is called VehicleLife (for iPhone).

 

[Loz]

I calibrate my bicycle computer by doing a roll measurement, measure in mm, one complete revolution of the front wheel and enter that measurement into the computer. It makes a significant difference if I'm on or off the bike. Tyre wear and tyre pressure also have a noticeable effect. If speedometers and consumption figures also rely on wheel revolutions, I can understand why they might not be accurate.


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That’s because tyre sizes are “nominal” if you put a new set of tyres on with 8mm tread depth and replace them at 2mm for example, then the rolling circumference goes down as it wears out, so the number of revolutions needed for the same distance increases. Also the speedo has to read high so the manufacturers cant be blamed for you getting a speeding ticket.

 

[Erbster]

There are several out there. Mine is called VehicleLife (for iPhone).

I like Road Trip.

 

[stephen watson]

That’s because tyre sizes are “nominal” if you put a new set of tyres on with 8mm tread depth and replace them at 2mm for example, then the rolling circumference goes down as it wears out, so the number of revolutions needed for the same distance increases. Also the speedo has to read high so the manufacturers cant be blamed for you getting a speeding ticket.

I agree and think my speedo over reads by 5 to 10%. So going back to my original question, if the speedo is dependent on wheel revolutions, presumably this means the odometer is out by a similar amount? Just interested to know because if so presumably it's even harder to match manufacturers mpg, assuming their figures are 100% accurate.

 

[Bellcrew]

I agree and think my speedo over reads by 5 to 10%. So going back to my original question, if the speedo is dependent on wheel revolutions, presumably this means the odometer is out by a similar amount? Just interested to know because if so presumably it's even harder to match manufacturers mpg, assuming their figures are 100% accurate.

There are so many variables, stop start city traffic when cold to long distance cruising at 60 ish on flat motorways in calm weather. The best you can hope for is a rough approximation.
After 10,000 miles my total average on the MFD is 42 mpg, I estimate it over-reads by about 10% from sample brim to brim measurements. That consumption is still far better than I anticipated when buying.

 

[alter]

today the engines consumed 250 liters / H ..... those of the boat ....... the engine of the Cali I do not even notice that it consumes!

 

[Amarillo]

There are so many variables, stop start city traffic when cold to long distance cruising at 60 ish on flat motorways in calm weather. The best you can hope for is a rough approximation.

Indeed, but if distance recorded on the odometer is dependent on wheel revolutions, like my bicycle, even brim to brim mpg calculations will be out due to tyre type, tyre wear, load and tyre pressure.

A more accurate mpg would come from GPS tracking and brim to brim measurements. GPS signal loss in tunnels etc would be a problem, but I guess a crafty computer could overcome this by calibrating wheel revolutions to the GPS data, and very accurately approximate during periods of signal loss.


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[Stoneybroke]

And then the heaters use some diesel throwing all the calculations out!
Even if you don't use the parking heater, the coolant heater kicks in below 5°C so uses a bit of extra diesel.

 

[Stoneybroke]

I use the Fuelio app on my android phone.
Just done a report and over the 18,403 miles since my first fill up, my average is 34.97mpg, whereas the average MFD consumption sits at around 39mpg.
The scary thing is it tells me how many £££'s I've spent on fuel!
Most of the fuel I use is standard Esso - my local tends to match the local Supermarket prices, plus Tesco Club are points gained.

 

[Meoncoast]

Our 150 gives better MPG than our 2012 180.
I never check MPG other than on the on board computer and only to check really to make sure it is about right. If I wanted a cheaper daily drive to run would not have bought a camervan.
Both van's MPG increased after 10k
although 150 beter than 180 mpg excludes extra spent on adblue

 

[Meoncoast]

anyone running on the red farm stuff

 

[Amarillo]

anyone running on the red farm stuff

Diesel for heating should be charged a different duty. I wonder if there is a way to get a duty rebate on the fuel we use for heating?


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[Bellcrew]

Diesel for heating should be charged a different duty. I wonder if there is a way to get a duty rebate on the fuel we use for heating?


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Indeed, pleasure boats in the UK use red diesel (a cause of disquiet and fines in Belgium), and duty is only imposed on the fuel used for propulsion, the amount used for heating is subject only to 8 % VAT. When I fill our boat up the calculation is 2/3 rd propulsion/1/3 heating. I doubt if the exchequer would be happy to use this formula for motor homes.

 

[2into1]

On the Motorway I tend stick to 55-60mph as going much faster seems to decrease the mpg significantly.

I know you’ve an auto Simon, but are you in top gear at those speeds? I’m surprised how highly geared by 140 manual is, and it seems more economical at slightly higher revs, despite the aerodynamics of a brick.

 

[SimonB]

Yes it is in top gear. We have the M5 going south from just after Weston that is totally flat. on calm days it is perfect to set the cruise control to a speed, set the fuel consumption display to actual and see what effects different speeds, AC on or off, winter tyres or the regular ones, etc have on the consumption. But I've not tried the Manual or Sport setting to see if forcing it to stay in 6th at higher revs has on mpg. I'll try that next time. Thanks for the tip @2into1 .
I realise that the fuel consumption display reads high, but as long as I am comparing readings from there with other readings from there, I think they are valid.
I try to drive in light shoes as heavy shoes tend to press down on the throttle if you are not on Cruise Control. I also lift my foot off the throttle every few minutes as the natural tendency seems to be for your foot to press harder with no perceptible increase in speed but an increase in fuel consumption.

 

[Borris]

anyone running on the red farm stuff

Premium super ultimate red or plain cooking red?

 

[SimonB]

Premium super ultimate red or plain cooking red?

Gran Reserva

 

[Ben Nevis]

Indeed, but if distance recorded on the odometer is dependent on wheel revolutions, like my bicycle, even brim to brim mpg calculations will be out due to tyre type, tyre wear, load and tyre pressure.

A more accurate mpg would come from GPS tracking and brim to brim measurements. GPS signal loss in tunnels etc would be a problem, but I guess a crafty computer could overcome this by calibrating wheel revolutions to the GPS data, and very accurately approximate during periods of signal loss.


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GPS does not loose the distance through the tunnel. Once out the other side and receiving the satellite again, it continues measuring the journey from the last point before you entered the tunnel.

 

[andyinluton]

But gps will only measure the distance across the ground & take no account of elevation changes, Ok in Norfolk but makes a difference in the Alps.

 

[Ben Nevis]

GPS should not be considered more accurate at measuring distance traveled than the on-board milometer.

Several factors can cause inaccuracy and sometimes wide variation in satellite readings but even when accurate readings are available, GPS only actually measures in straight lines from point to point and doesn't take account of the curves or undulations on most roads. This is probably why most cycle race distances are measured with a wheel rather than GPS.

I would think the most accurate and practical measure of consumption for most would be fill to fill with a correction for milometer/speedometer calibration. In my case this shows the speedometer (and therefore milometer) reading 5% faster than GPS based on GPS speed measurement at 70mph on a very straight and level road.

 

[Ben Nevis]

But gps will only measure the distance across the ground & take no account of elevation changes, Ok in Norfolk but makes a difference in the Alps.

Actually that is incorrect on both points, it doesn't measure across the ground but it does take account of elevation changes.