BF 2007 falcon factory LPG uses 40lt gas per 100 km I have had it tuned and am told all ok can I convert this to Petrol

no country told , matters! for sure no USA. KM speeds tell that. GUESS Australia????
down under IRON./? or the exported Austrilia to Africa.???

won't rev out (means RPM limited,means low power from engine. so means engine power is lost.
and NOT:
not transmission slip. (none told A/T or stick?)
good post, lots of causes for any kind of engine power loss
I will not pretend to guess it but in terms of physics yes.
engine has mech problem , cam belt slipping then changing back.
engine low power.
does 4LT mean 5liter engine, or 4L +LT MODEL CAR
The BA sub model falcon? IDK
wiki says

• 4.0 L Barra 182 I6 (petrol)
• 4.0 L Barra 240T I6 T (petrol)
• 4.0 L Barra E-Gas I6 (LPG)

seems there are 3 engines used, day1 sold,
gas Petrol
turbo petrol (gasoline)
and LPG (propane) gas.

deep wiki' me.
182 is 6 cylinder DOHC cam engine with pesky VVT valve timing
chain drive cam +VVT
VVT can stick so there is my first physics IDEA on engine problems.
or stick head valve.

the classic list, (means what is tested in order.) using tools.
1: good engine, means all tests pass,and VVT'
2:good spark ever do a tune up in 20 long years,spark + filters.
3: bad fuel, or bad fueling rates. , fuel pump on lasts legs. sure.
4: egr struck open ,even random they do.
5:;random bad wiring to all that.

that is my best, 12,000 miles away and blind, cant see car engine or transmission there.

same car family , same cluster

with not knowing what engine is in your vehicle its not easy to answer but normally the plug gaps can stay the same has a gas/petrol engine normally 1 to 1.4 mm if you have got the owners manual then use the plug gaps in the manual a slightley larger gap can give a small advantage but if it runs ok on standard gaps leave it alone

dedicated gas ( LPG gas only) has no filter as the line is from the tank to the converter direct
what has to be monitored is the converter as they corrode internally and play up
If you have liquid direct to injectors then the injectors have to be serviced as per schedule

have an automotive gas expert check the converter for the gas
There is a problem in the gas supply from that unit

as lpg expands to around 200 times the liquid state it will be difficult to calculate as how you want it
the car has a liquid tank but the liquid is converted to gas in a converter on the manifold.
basically from experience an engine on lpg gets about 75% of the mileage as if it was on petrol for the same amount of lpg in litres
talk wit an accredited specialist automotive gas fitting shop and they may be able to give you a better idea
you can work it out by filling the lpg tank to shut off and setting the trip meter to zero and then when you have done say 100 klms refill the lpg tank to shut off and that will indicate how many litres were used in that 100 kilometers

Without knowing the car.It should use about 16lt per 100km.No van.Just moving around.Ok.The van is going to use more.That model does use fuel but not as much as you quoted.You said no towing its about 21lt.Thats cause there is too many traffic lights in melbourne.:)I'am from Gold Coast.The injectors can be the cause of bad milage.Dirty or playing up.A bad air filter is a common problem.Because you said it was a prob. on fuel and gas,I would start looking at the inlet side of things first.The filter?Yours is efi.Get the inlet flow meter checked and then the exhaust oxygen sensor.If one is out,it makes the other do the wrong thing.Like pump more fuel.I hope this is of help

areas to check are the fuel pressures, fuel hoses, fuel crossover solenoid
It indicates that the ECM is receiving the correct information from the sensors to adjust for LPG but when crossed over to petrol signals from sensors are not as clear as they should be
This can be from a power drain to components caused by a faulty solenoid or relay that is reducing the voltage available to the sensors ( normally in the air intake system or the heated oxygen sensor in the exhaust manifold )
check all harness connections and wiring
if you are checking it all your self then a workshop manual that has all of the voltages for each sensor would provide help

Alittle more info please.What tests/checks have you made ,what parts have you replaced when was vehicle converted and what system was used Factory/aftermarket.

check that your cooling system if full of coolant. the coolant that keeps the engine cool actually keeps the gas converter from freezing. alternatively is it hard to start of a morning when cold, if so could be butane wax build up from the gas, in the convertor.