I have had LPG fitted and expected the fuel consumption to be thrown out when running on LPG but this does not spear to be the case.

Does anybody know how this works as I can't see how it can measure when on LPG.

cant answer your question directly but the consumption figures for LPG are very similar to Diesel , so in real terms it wont change that much , its just the big price difference that makes people say its twice the mpg, its not, its just half the price.

as for the calculation, not sure how LPG would affect this as im sure the Onboard diagnostics measure diesel through the pump or injectors and versus this against distance travelled

That is what confuses me.

I was expecting the consumption figure to just keep rising as no petrol would be used but mileage would increase. This has not happened however.

how does the LPG get into the engine? through the original injectors or is it fed in to the fuel system before that . I guess if you reset your mpg reading and you start getting similar looking numbers to petrol then the system is now reading the lpg , as I said , its similar to petrol ( not diesel as I prev said) the point of measue must but at a point after lpg gets into your fuel system

The car still thinks its running on petrol as the counts the duty cycle of the petrol injectors - that's how it calculates the MPG Stolen / Gone: 2006MY Supercharged LPG Autobiography - Zambezi Silver with Sand/Jet/Jet Oxford Leather.....

The engine management ECU calculates how long it needs to open each injector to get the desired (calculated or actually taken from a pre-programmed table) amount of fuel into the combustion chamber. It uses sensor information in combination with the table (multi dimensional) to determine how much fuel is needed to get the optimum mixture (amount of air going in from the MAF, air temperature (i.e. density), Lambda sensor data (mixture too rich or too lean or just right), etc.).

It then opens each injector for the determined amount of time and combined with pressure and injector dimensions, that gives a certain amount of fuel going in. The calculation is within approx. 10% of the real fuel usage apparently.

Above info comes partly from my engineering training and partly from a friend that used to work for JLR as a powertrain engineer (i.e. he programmed these ECUs amongst others). MY 2010 5.0 SC Galway green and sand interior!!
Have the Faultmate MSV2 Extreme to be tinkering with the settings etc. !!

ebajema, that was exactly my understanding and the reason I cannot understand people who say the trip computer mpg calculation in not accurate - it simply has to be! 2007 Zermatt Silver TDV8 Vogue SE - now sold but was a great car!

I thought that an LPG installation uses its own injectors that are drilled and tapped into the inlet manifold, I also thought that the installation had its own ECU that is a stand alone system from the main cars system

Graeme, I don't understand either why it is not more accurate but according to my friend it isn't. MY 2010 5.0 SC Galway green and sand interior!!
Have the Faultmate MSV2 Extreme to be tinkering with the settings etc. !!

I expect it's accurate with regards to the amount of fuel used, but may be less accurate with regards to the distance travelled.

Assuming it uses the speedo to get the distance, this can be affected by different wheels, mis calibration, and even whether you have 4 brand new tyres or 4 nearly worn out ones. The smaller they are the more they have to go around for the same distance.

I've never normally found the MPG to be too far out in any modern car anyway. There isn't really a scientific way to test it, the fill the tank method and drive with a GPS distance measure isn't that accurate either, it doesn't work in tunnels, and the amount of fuel you put in can vary depending on it's temperature.

Any error in the calculation of distance by the vehicle would be present in both the computer calculation and any manual fuel used divided by distance travelled calculation.

So far, per ebajema's post, the trip computer should be accurate to 10% (as an aside, I'm amazed the error margin is this big given the method of calculation!) 2007 Zermatt Silver TDV8 Vogue SE - now sold but was a great car!

Unless you use a portable GPS to measure the distance travelled (as some do, when they do the full tank/drive/full tank method)