Regarding the tax man mentioned above..
I remember when Gasoline was less than $0.25/gallon (Oh were those the days)
Some years ago I pulled into a Marathon station (On Telegraph RD in Redford, TWP, (MI) just north of I-96 (I think the cross street is Arcidia) and there was a sticker "You are paying 87 Cents in tax on each gallon)
And the taxes have gone up since.
As someone up-thread said, Propane is a nice fuel but you need a lot of it 4 gallons of Propane =about= 3 gallons of gas. You need large heavy tanks to hold it (Adding to the weight of the vehicle) and in the event of an accident it is much more dangerous should it leak out. Not that gasoline is not mighty dangerous if leaking itself.
Now Natural Gas is a reasonable choice, but the same statements (save for the ratio) apply.