It depends and awful lot on the condition of the engine, especially the valve timing.
A good one will better 15mpg on petrol, just ask Johnny Boaterboy! on lpg you will either loose power or economy, or a combination of the two.
Our 3.9 (Blue) does about 12.5 on LPG but is noticeably less powerful, it's an old draw-through single point system with no feedback. It helps to have an ignition advance processor fitted but ultimately I have to switch back to petrol for top power.
My LSE runs a bells and whistles multipoint, I notice no loss of power on LPG at all. On a bad day it will do 9.5 MPG, however you drive it. With care it will do 12.8mpg, it's decated with a sports pipe, K&N and Iridium plugs. It needs camshaft, oh and it's a 5.0TVR V8 :twisted:
So the answer in short is "less than 20".
