MPG on LPG BMW 645CI

[sergeuk]

Hiya
I have recently converted my bmw 645 CI 4.4 V8 to LPG (Prins VSI). MPG around a town (central London with alot of traffic etc) about 15 was 16 on petrol. But only 20 MPG on the motorway, it use to be about 33 MPG on petrol. Is that normal ? I have done roughly 1k miles so far and will take my car back for checks etc soon.

Regards

[sergeuk]

Just wanted to add, those figures not taken from the onboard computer but calculated by how many miles i do per full tank of LPG/Petrol. Onboard computer giver 13(LPG) 17.5(Petrol) in town. Motorway 27(LPG) 36 (Petrol)

[rossko]

If the onboard computer is saying significantly different things about MPG for the two fuels, the gas system is out of tune. I'd say its lean by your figures.

When its working properly, the trip computer should report similar MPG (even though it won't be true!)

[sergeuk]

Trip computer almost correct when I drive in town. But on motorways is way off. I will go to the installer netx saturday and ask him to readjust.
Tnx

[billynoband]

Don't know if it's exactly right but I have found on both our cars that lpg gives nowhere near the huge mpg improvment on modern cars on a long run as petrol does. In very cold weather it's the same, I don't know where my lpg dissapeared to when we had all that snow. Must be the lpf faireys. Around town it's good though.

[sergeuk]

Yea. Definatly saves alot of money around town. I had bmw 118D with start stop thingy for a week to drive around a town it did 33 mpg for that week (that is 2L diesel). Now i get 15 mpg on lpg wich is = 30 mpg moneywise.

[mat_fenwick]

I'm no expert, but I do know that some systems also make use of the petrol fuel system beyond a certain throttle opening so maybe that is happening on the motoway? I wouldn't expect a BMW 645 to be exactly strained at normal UK motorway speeds however, so I think that's unlikely to be the reason, but still a possibility. To my mind that kind of system seems like a bit of a bodge anyway!

[sergeuk]

I did not notice any pertrol being used. I had car for over 3 weeks done 1200 miles and it did not even used 1/10 of a tank.

[Gilbertd]

Trip computer almost correct when I drive in town.

But you said it's showing 13 mpg when on LPG and 17.5 mpg when on petrol? It should say the same no matter which fuel you are running on. It's taking it's reading from the injection pulse times which are adjusted by the petrol ECU from the output of the lambda sensor. When you are running on LPG, those times should remain exactly the same with the LPG ECU adding the correction factor. If it is reading different, the correction factor is wrong and the petrol ECU is having to adjust the times to keep the mixture correct.

Some of the cheaper systems either add petrol or switch to petrol completely at full throttle or above a certain rev limit. A Prins system shouldn't need to do this and it will only do it if you have programmed it to. If you haven't told it to do it, it won't be.

I seem to recall reading somewhere that the older single point systems gives better economy on a run but are worse around town compared with modern injection systems. I don't know how true this is and my experience with modern systems is very limited but I can confirm that it seems right with regard to my own cars. The Saab for instance will do around 270-280 miles on 46 litres on a long run but can go as low as 170 miles on the same amount if used for lots of short journeys.

[sergeuk]

Ok I see what you mean now regarding trip computer. It is definatly showing different on petrol than on LPG. This Sat will take it to the installer and hopefully they will put it right. My mpg around London close to your Saab

[billynoband]

I would not worry too much as you have a top quality system fitted to a great car. Both petrol and lpg running needs to be correct though. Don't believe any fuel computer
I have noticed others say this car will do 30 plus mpg on petrol as I looked around when I thought about buying one (ended up with a Zafira, don't ask).
Speaking honestly I don't see how any auto V8 is ever going to do more than 25 to the gallon even on a 40mph run. If you think about it how is any V8 ever going to do more than 15-20mpg average with all those cylinders and weight even driving slowly. Do some run on 4 cylindes when going slowly like the Chrysler Hemi?
I recon 15mpg on gas is good for a auto V8 but that's just me. Good luck, I want one.

[sergeuk]

I think 15 MPG is very good around town but on the motorway 20 MPG is a little disapointing, will take long time to save money I paid for the conversion (2k). Cheaper system was only £300 cheaper so there was no point in it. Oh well what is done is done. This car does 30 MPG on petrol because of the engine is made of aluminium and some of the body made of plastic and it is fairly light only 1600 ish kg. I had small IS220 lexus before and it was over 2000.

[miker33]

I think only 20mpg on a run is a little low, my 740 (okay slightly older engine design but still very similar) will do approx the same 15mpg on LPG round town (16ish on petrol) but gets around 22mpg on a run (28ish on petrol). I expect that the higher 30mpg+ on a run with the 6 series on petrol is more down to the aerodynamics as the weight doesn't make much difference once cruising and the 7 series (E38) is a brick in comparison to the new 6 series so really you should be getting around 24mpg on a run.

[pyro225]

Did you have any result on this? I have just posted and noticed your post, i have an m5 which does about 19-20 mpg petrol but 12-13mpg gas, hardly worth using...

[sergeuk]

I had my system retuned checked etc. but mpg did not improved much only a little maybe 1 mpg maybe less.
Well M5 have 5 litre engine right? . My engine is 4.4 I get 15 ish mpg in town so yours will comsume more I guess, so if u get 12-13 in town I would think it is ok. I never had an m5 but my friend had m3 and in town he got around 16 ish nad around 20 ish on motorways. Did not trust MPG on trip computer, fill up full tank run it to the empty and calculate that way.

[tommo2009]

When they retuned the vehicle, was is stationary in a garage or did they tune it while driving.
My X5 had similar issues and it was only when I got someone to drive it and I retuned it did the MPG come up.

A lot of the time the systems will autocal and it will run fine and give good MPG other times you will need to tune it yourself looking at the live data feed from EOBD

the X5 on a run now does 21-22 on pretrol and around 18-19 on gas.

[sergeuk]

to tommo2009,
Do u get that MPG in busy town or motorway etc.? What is the engine size? I get 22+ MPG ish easily on motorway if i dont go over 70 MPH much and there is no traffic jams. But on petrol it is over 30 with above conditions. If i drive about 60 MPH than MPG goin up to 25 ish. But who want to drive that slow on 333 bhp

[tommo2009]

Its a 4.4 X5, used it on the weekend and it was returning 23MPG on LPG on a run of 200 miles.
Not sure what it is doing around town but it does not seem to be excessive.

30mpg is extremely good for a 4.4 V8

[pyro225]

Serge, need some advice, looking at getting a 645 now the new shapes out these are getting closer to the 10k mark and i can sell the m5 add a few grand and be the proud owner of a lovely 6. however is the conversion worth doing? for a money perspective what do you get out of it? as id want to convert it and is yours auto? my currently costs £55 to fill and will see me 250-300 miles depending ifmixed or solid motorway miles. what do you get out of yours for how much? your advice will be much appreciated

[sergeuk]

Hi Pyro
My car is auto. I get 250 -260 miles around town and 350 miles motorway only. I get about 78 ish liters in my tank. Worth converting in you do more than 10k miles and will keep your car for 3 years to pay off conversion plus save a little. That is if price difference will stay at 50-60 p.