P38 MPG calculation. How's it done.

Don1 · Aug 18, 2009

Hi All,
I have a 2000 P38 4.6 with multipoint LPG conversion. I always assumed that the average MPG stated on the dash computer would be of no use as running on LPG but noticed while playing today that it does seem to measure something meaningful. Reading drops when going uphill and raises when cruising etc.
If the system was measuring petrol use I would not expect this to change when running on LPG as no petrol is being used. So now I'm wondering where and how is the fuel consumption measured and does it actually reflect LPG use.
Idea's anybody.

Don.

gavbriggs · Aug 18, 2009

it is supposed the measure the rpm, and miles travelled then work against a table to come up with what mpg you should be getting.

now, the good bit is that its only a guide as it doesnt measure fuel used over a period of time but it can calculate how many more miles you can do on the remaining fuel.

the extra good bit is how it arrives (or is told to arrive) at the figure it indicates to you, the driver.

do you remember those paper things and you write numbers and colours on em then ask your friend to pick a number then pick a colour etc. well thats the table it uses!!!

Don1 · Aug 18, 2009

Ahh, So no point getting all excited about my reported average of 21 MPG then.
Thought it was too good to be true.
Cheers Gav.

Don.

gavbriggs · Aug 18, 2009

you can get excited as its all relative, if say your avg was 21mpg on the clock but it was 19 in reality then if it went to 22.3 (my personal best) then the actual moves to 20.3.

and to be honest the manufacturers always massage the figures!!

best way to calculate it is to count the miles to the tank!! even with that tho you have to consider driving style, road type etc.

on gas tho you'll notice the same ish mpg (on mine about 3mpg less than clock) on long jouneys but because you are cruising around 2000 rpm you dont notice the drop in power from the lpg (lower calorific value) around 20% lower!!

lpg is definately worth it, 47.9 ppl where i go, versus 101.9 ppl for petrol!!

hammy2909 · Aug 18, 2009

gavbriggs said:
you can get excited as its all relative, if say your avg was 21mpg on the clock but it was 19 in reality then if it went to 22.3 (my personal best) then the actual moves to 20.3.

and to be honest the manufacturers always massage the figures!!

best way to calculate it is to count the miles to the tank!! even with that tho you have to consider driving style, road type etc.

on gas tho you'll notice the same ish mpg (on mine about 3mpg less than clock) on long jouneys but because you are cruising around 2000 rpm you dont notice the drop in power from the lpg (lower calorific value) around 20% lower!!

lpg is definately worth it, 47.9 ppl where i go, versus 101.9 ppl for petrol!!

My personal best was 28mpg. Straight after I'd reset it, I was doing 40mph for about half an hour stuck in motorway traffic. Never had it that high again since

Don1 · Aug 18, 2009

If your in the area
Calor Gas
Shawfield Rd, Barnsley,
S71 3HS
41.9 p/Ltr

I always top up when passing.

gavbriggs · Aug 18, 2009

theres also a shell in doncaster that is very cheap, trouble is the distance from rawmarsh is bit much to make a significant saving, but whenever i'm going there i run it as empty as possible!!!

phantom21 · Aug 19, 2009

The way MPG is calculated is
The ECU knows the fuel pressure
the ECU knows how much fuel the Injectors flow when open
The ECU keeps a record of how long it has told the injectors to open
This is then used against distance travelled to calculate MPG
But when the engine is running on LPG the whole calculation is meaningless becauase the ECU is fooled into thinking it is still running on Gasoline

nathan5346 · Aug 19, 2009

the kids cleaned me car out last week... and threw out me lpg filling station book ffs

gavbriggs · Aug 19, 2009

nathan5346 said:
the kids cleaned me car out last week... and threw out me lpg filling station book ffs

you can go to a site, think its called get lpg. or get the overlay for tom tom!

gavbriggs · Aug 19, 2009

phantom21 said:
The way MPG is calculated is
The ECU knows the fuel pressure
the ECU knows how much fuel the Injectors flow when open
The ECU keeps a record of how long it has told the injectors to open
This is then used against distance travelled to calculate MPG
But when the engine is running on LPG the whole calculation is meaningless becauase the ECU is fooled into thinking it is still running on Gasoline

yep thats what i said!!! innit?:doh::doh::doh::blabla::blabla::blabla::blabla::blabla::blabla:

kooky_guy · Aug 19, 2009

gavbriggs said:
you can go to a site, think its called get lpg. or get the overlay for tom tom!

You have to take the TomTom overlay with a pinch of salt. It directed me to two lpg/petrol stations that didn't exist while we were on holiday in Cornwall - to be fair they did exist once, but obviously a long time ago - one was a now a bungalow and the other was a caravan park!

Most frustrating having to use the expensive stuff!

Guy

P38 MPG calculation. How's it done.

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