Well take me and my brother in law, he got his minivan converted, so we've both got minivans that nominally in the hands of the general populace get 18mpg combined... With his converted, he pays about 2/3 the price of gasoline to fill it, but gets about 15mpg equivalent, but he's 3mpg ahead of the 12mpg that the difference in cost would indicate, or 3/15x100=20% ahead.. and went about $2000 in the hole for it, so it's gonna be a few years in payback when he does about 200 miles a week. Me I run gasoline, and have a few simple mods, and I'm at 24mpg, 33% ahead... with very little capital outlay. A couple of times last year though, the natural gas gas filling station price went high enough that he would probably have been better off putting gasoline in.
There used to be more "factory" CNG vehicles available up here, but they seem to have replaced them all with flex fuel, which sucks when we've only got a handful of E85 stations, in the whole Province and the gubbmint is giving cashback on 'em whether you can fill 'em or not.
Ideally a factory CNG or conversion prospect would be a high compression motor, in order to avoid losing so much power and mileage per gallon equivalent with the conversion.
I remember The RoadWarrior..To understand who he was, you have to go back to another time..the world was powered by the black fuel & the desert sprouted great cities..Gone now, swept away..two mighty warrior tribes went to war & touched off a blaze which engulfed them all. Without fuel, they were nothing..thundering machines sputtered & stopped..Only those mobile enough to scavenge, brutal enough to pillage would survive. The gangs took over the highways, ready to wage war for a tank of juice