my MJ on propane gives round 20-22 mpg on the highway, some less in the city, but propane is 40% cheaper, so you must take that into account
the 4.0 is rated at 17 mpg when running on gasoline so i´m not doing bad
you also save a lot in oil changes, i have not changed the oil in 2 f***ing years and still looks clean, i´m changing it the next weekend because the additives are probably long gone but 2 years? that´s like 5 or 6 oil changes if it was running on gas, all that money can go into propane instead...
of course some sacrifices should be made, i´m running on the lean side most of the time, running at no more 2000 rpms, after that it starts losing power, but when it comes to torque i didn´t notice any change, even fully loaded it doesn´t sweat as long as i keep it in the 1600-1800 rpm range
unless you install and tweak your lpg system yourself you´ll end up spending more, most lpg mechanics give the engine too much propane to mimic the response as if was running on gas