Petrol / LPG - Am I missing something?

James_N

James_N

James_N

Soldato
Joined
25 Jan 2003
Posts
9,497
Just to update, it seems my car is more economical on petrol than it is on LPG!!

Ive done 150 miles, used around 45 litres of LPG, at 41ppl. Using http://www.torquecars.com/tools/uk-mpg-calculator.php, doing 150 miles on 45 litres of gas at 41ppl works out at 15.15MPG! This is around town.

Now on a full tank of petrol (around £43 to fill it up), i can do at least 230 miles to a tank around town, or as much as 350 miles to a tank on a steady run. Assuming £43 is around 45 litres of fuel, that works out at 23.24mpg around town, or 35mpg on a run.

Am i missing something, or is my car more economical on petrol than LPG?!
 
Id get your LPG setup again if I were you. Doesn't sound right at all if you consider the energy densities of each fuel type.

EDIT: Just re read it. Doesnt sound too far off TBH. The point of it is to lower pennies per mile, not increase MPG, its a less dense fuel for a start anyway. I borrowed a van once, it was better in 4th than 5th cos the LPG map at low revs was pretty poor. It can take some work to get them running the correct air fuel ratios if you're using the std ECU still.
 
Last edited:
The chap where I will up at the LPG station seems to think i should easily achieve 250 - 300 miles to a tank of LPG, but at the rate the green lights go down on the dash, this does not appear like its going to be the case!

I do need new injector seals, as the car is ticking over very lumpy on gas, but i cant see that affecting the MPG so much?
 
James_N said:
The chap where I will up at the LPG station seems to think i should easily achieve 250 - 300 miles to a tank of LPG, but at the rate the green lights go down on the dash, this does not appear like its going to be the case!

I do need new injector seals, as the car is ticking over very lumpy on gas, but i cant see that affecting the MPG so much?
Click to expand...

No it wont 43 litres of petrol will always get you further than 43l of LGP. Looking around the net seems LGP decreases MPG 15 to 20%
 
LPG is higher octane, but of a lower calorific value than petrol IIRC.
The car should be in the region of 10%-15% less economical on gas, but the gas price being approx 50% that of petrol should considerably offset that loss.
 
LPG will always be less efficient, but shouldnt be by the margin you suggest in the OP

my 525 gets ~18mpg on LPG and about 23mpg on petrol (petrol figure maybe inaccurate due to rarely doing any great distance on the stuff)

also depending on setup the rate the green lights go down are fairly irrelevent, at full itll take forever for the 1st light to go but the next 3 go pretty quick, however the red light on mine will last a good 60miles (this however is an 80ltr gas tank so a long red light run is to be expected), best bet here is to litterally run the gas tank till its empty (have enough petrol ready so you have something to kick over to of course) so you can really see how far a full tank will take you, or even how far the red light lasts, you might be surprised, at the end of the day you do have a fairly average gas tank though

as for lumpy turn over, also consider things like external temperature, in the cold certainly gas is very irritable and generally sluggish/no power at all till the engine gets towards its ideal operating temperature, only really an issue if, like me, you start on gas
 
Ive spoke to the guy who installed my kit and he said the mixture might need adjusting. He will have a look at it for me.

Im not too keen on literally running out because of potential of backfiring issues and blowing the airbox off!
 
fair play, the airbox has blown on this one before, however, according to my grandad, who previously owned the car and had the system fitted, this was due to one of the numerous valves giving up as opposed to abuse of the system
 
I heard that old volvos (And probably other cars too) are not so hot on LPG because of the single point injection. Newer multi point engines are much better.

How much was your conversion? I have an 850 estate on 185k and would probably go for it on less mileage but don't really think it's worth it.
 
LPG simply gives worse mpg, in the Corolla it's up to 20% worse than on petrol and in the C5 about 15%... A non issue usually though as lpg is around 45 cents/liter atm. Nearly 3x as cheap as petrol.


Also (provided you have a g3 gas installation), the car still has quite a lot of lpg ( think enough for at least 50 miles iirc) when all the green lights are out and just the red one shines, once the thing beeps and switches to petrol only then your lpg is truly running out ( it beeps & auto switches to petrol when it gets too little of it for a sec), switching back to lpg after the beeping/switch to petrol still gives you bout 5 miles to drive on lpg ( provided you don't floor it/ use throttle sensibly).
 
Last edited:
so LPG wouldnt be as good with a SFI injection system (single point) on my FiestaST 2litre Duratec HE?
 
James_N said:
Am i missing something, or is my car more economical on petrol than LPG?!
Click to expand...

If you have one of the old style systems with a mixer ring or single point injection then they are pretty poor for both economy and power compared to the new multi-point systems, and much more prone to damaging back fires. That said it does sound like it needs setting up correctly which will improve economy.
 
Overlag said:
so LPG wouldnt be as good with a SFI injection system (single point) on my FiestaST 2litre Duratec HE?
Click to expand...


The LPG system and the original fuel system are completely separate, they don't use the same injectors. Multi-point LPG systems are always the best solution if available.
 
You must log in or register to reply here.
Back
Top Bottom