With LPG having a lower Calorific(!) value than petrol, you actually get less mpg!
The main benifit of LPG is it's vastly cheaper price,in the UK around half the price of petrol.
(Untill Brown starts pushing up the duty on it !)
Plus there are enormous drawbacks for a bike-kit.
It needs to be stored in a Pressure Vessel (think Calor gas bottle),the weight of which is considerable.
Then there is the actual kit needed to feed the carb/injectors.
And finally the availability of LPG away from "civilised" area's
I'm sure others will give more details, but it's a no-no at the moment