more ?s about tires

[Bob Trevithick]

Hi gang,

Just wondering if someone could clarify something for me.  The Tire
Rack person I spoke to suggested a set of 15 inch snows despite the
fact that my summer wheels are 16 inch.  Specifically, they suggested
195/65QR15 Blizzak WS-50 tires mounted on steel wheels.  My '01 A4
sport package came with 205/55ZR16 Dunlop SP8000E tires.

Is the outside diameter of these two tire/wheel combos the same?  The
Tire Rack guy said no problem.. but I want to double check before I
mount these.  Can anyone point me to a good URL which explains what
all these numbers actually mean?

A Quattro with serious snow/ice/mud tires should be just about
unstoppable, I would think.. although the old joke may apply: with 4
wheel drive you can get stuck so badly that nothing short of an Army
helicopter can get you out.  ;-)

Thanks,
Bob