The fuel consumption will still be the same as if you are running on petrol or LPG.You will still burn the same amount of fuel to drive 1 mile down the road irrespective .The only difference is that the unit cost per mile travelled is less.
Lpg is reconned to be about 10% less efficient than petrol so on LPG your fuel consumption will be worse than petrol but overall costs will be less.
I would expect 12-15 mpg or a lot less if you drive it hard.