LPG FILLING PROBLEMS

I would like to know certain things regarding LPG as auto fuel for piaggio auto rickshaws (piaggio ape city). The auto is supposed to have a claimed average of between 16-18 but only returns 10-12 and even after getting help from vanaz engineers (makers of the lpg gas kits ) the average actually decreased, another method that i used was to reduce the amount of gas injected into the the system so as to reduce rpm as well as speed leading to increased average ,but is there another way to increase its average without decreasing performence. Also im facing the above mentioned problems such as filling stations filling more gas than the actual limit and not being able to fill gas unless the auto has run approx 100km 

I would like to know certain things regarding LPG as auto fuel for piaggio auto rickshaws (piaggio ape city). The auto is supposed to have a claimed average of between 16-18 but only returns 10-12 and even after getting help from vanaz engineers (makers of the lpg gas kits ) the average actually decreased, another method that i used was to reduce the amount of gas injected into the the system so as to reduce rpm as well as speed leading to increased average ,but is there another way to increase its average without decreasing performence. Also im facing the above mentioned problems such as filling stations filling more gas than the actual limit and not being able to fill gas unless the auto has run approx 100km 

PLEASE HELP......

hello all! please help me in solving my problem. i plan to purcghase  spark ls lpg, duo car next week. teh lpg tank capacity is 26 kgs. but the pettrol pump mentioned that they sell it in kgs. now what is the conversion from litres to kgs. how much is my tank capacity in litres therefore