you may not necasarily burn more lpg than fuel, an engine set up to run lpg only and tuned for lpg will equal or even better petrol burn.
to start with gas will burn at 11 or 12 or even more to 1 with no addatives, gas specific cams are available, voodoo cams i think there called.many other advantages from gas only too like its already gas and burns freely, problem with most conversions is may sacrifices are made to have motor run on petrol too.