Mate, down here in Oz there are heaps of cars running around on LPG (Liquefied Petroleum Gas, which is basically a propane-butane mix). Ford Australia makes a version of its 4.0l inline six which puts out 200 hp on LPG (the petrol version does 250 hp).
The most frequent users are taxi's and government departments because they don't really care about the drop in power but love the fact that LPG is a fair bit cheaper that petrol.
I haven't seen any police cars using it though. They probably don't want the drop in power.