LPG Fuel Consumption
The same base-engine (i.e. a converted petrol engine), will always use more LPG than petrol and this is because the petrol and LPG have different energy contents.
LPG has about 24.4 MJ/litre of energy, whereas petrol has about 32 MJ/litre. As such LPG has only 76% of the energy of petrol per litre, or another way to look at it is that 1.3 litres of LPG is needed to have the same energy as 1 litre of petrol.
Therefore to match the fuel consumption of a petrol engine, you need to have a thermal efficiency that is 30% better.
This cannot be achieved by just changing the fuel from petrol to LPG, no matter if it’s liquid or vapour injection.
There are some efficiency advantages by injecting LPG over petrol and this means that you should expect to use 20% more LPG than petrol. Of course it all depends on the vehicle being converted and the driving style.