What level of autonomy does a converted LPG vehicle have?

His will depend on the tank capacity. Every tank has a maximum volumetric capacity on average of between 30 and 100 litres. If a vehicle consumes x litres/kilometre on petrol, it will generally consume 1.20*x litres/kilometre on LPG, which multiplied by the tank capacity gives the level of autonomy for LPG-driven vehicles. The higher consumption in litres of LPG (on average +20%) compared with petrol, is obviously more than compensated in economic terms by the lower price of LPG at the pumps, making it markedly more advantageous.
Finally, it has been noted that tanks may be cylindrical or toroidal. In the latter case it is mounted in place of the spare tyre and consequently does not reduce baggage carrying capacity: the spare tyre can thus be managed as any other piece of movable baggage or replaced with an emergency kit, something that is now increasingly common also on new cars.