Raul writes:
> ET or offset is dependant on the width of the wheel. The wider the wheel,
the lower the offset number-given in milimeters.

This is not true.  Offset is completely independent of wheel width.
However, the offset you choose to use on your vehicle will need to take the
width into account for clearance.  Raul correctly gave the definition of
offset (distance between the centerline of the wheel and the mounting face),
so it's best to draw this on a paper to fully understand it.  Think of the
offset as determining where the center of your tire will be.  Then from
there you need to figure out if the outside of the tire will be under the
fender, or if the inside of the tire will be rubbing on the suspension, etc.
There's a good diagram of offset at http://www.comp.co.uk/frm/technical.htm.
There's another explanation of it at their US site,
http://www.usacomp.com/Offset.htm.  Note that I do not work for
Compomotive - I'm just using their site as a good source for an offset
diagram.

As Raul pointed out, there is only so much room from the mounting face to
the suspension parts on the inside.  Thus, to "fit" those really wide wheels
(over 7 inches), the offset starts to get lower to put more of the wheel
outside the hub face.  Eventually they'll stick out too far, and you can
have problems like your sliding door hitting them, the fender lip hitting
the tire, etc.

It's pretty easy to make an Excel spreadsheet to calculate how much of the
wheel is inboard and outboard of the mounting face.  I made one and use it
any time I'm considering different fitments.

Chris