assuming the gas is not going into the stratosphere, then all the consumption should be down to the setup. now, on mine, a leonardo controller is set up by software, this regulate the amount of fuel that is introduced. as i use it mainly for plodding there is no need for it to consume vast amounts of fuel. it is set up fairly well as i have had a good play around with it to get familiar and i wanted to cut out the backfiring on over run.
the lpg forums is a goldmine of info if you wanna check it out. a fella called rossko is a bit of a guru on there and i think he's a landy enthusiast too!
lpg is just like setting up the carbs on a normal engine, fuel and mixture. there is also the software side but its really not that complex!! i would suggest you get hold of the lead for it and the software its only about 15 quid and can save that in fuel.
i got from rotherham to ashford in kent and around ashford for a bit, dunno how many miles, then to around gravesend on the a2 to a filling station on approx 95 litre fill up that works out around 15mpg which doesnt sound too good but given that some of the drivng was town, some of the motorway was traffic, the rest of the motor way was at speed (70+) and not at the 56mph that LR state as the optimum fuel economy speed all for around 45 quid in a 2.7 ton car with a 4.0engine. i thort it was fecking amasing!!
it'd have cost me over 100 pound for that in petrol!!
rule of thumb is around 15% less mpg than petrol but at around 45% cheaper per litre youre 30% better off, not the 50% that most people state on fleebay!!!
incidently the car indicated 21mpg on the way home but i had to get off the motorway, if i'd continued to, lets say the north pole, then i would have een in proft i thnk!!!