This has already been overcome by Prins for some manufacturers like Audi and even GM direct injection engine, they do a direct injection lpg liquid system which uses the cars original fuel injectors, does away with the inefficient vapour systems currently in use think its called liquigas V2 or something similar.
I am aware of that, but have yet to see a successful install in the UK. And current prices are .. lets say not really affordable.
Other issues:
-No V8 version available so far, so supercharged Range Rovers and the like are not convertible.
-Most systems are manufacturer specific, so one size doesnt fit all, like they currently do.
-As far as I can make out, the systems need to inject a small amount of petrol to keep the injectors from getting damaged. So the so called current saving of LPG will diminish.