This is a common conversion in europe where gasoline taxes
are much higher than LPG.

The conversion includes a carburetor kinda thing that feeds
into the intake (commonly -- there are actually several ways
of doing this) so some cars run on both LPG and gasoline.

LPG will get you far worse MPG and power than gasoline
because it simply has far less energy per unit volume than
gasoline.

If you could use natural gas and could get it from your
house you would subvert the tax system that makes some fuel
more expensive than others. This *might* make a converison
worth while. If your car has fuel injection I think you can
add one of the LPG delivery devices to your intake and
still run the gas system (use a switch to turn one system
off and turn the other on). The advantage of doing this is
that you can still get from place to place if you can't
find LPG. The drawback is that you will be losing trunk
space and adding cost.

Biodiesel is a better alternative fuel but it isn't worth
the pain to convert a 164 to diesel -- you'd be better off
starting with a MB with a turbo diesel and make your own
biodiesel.

chris