I have been doing lots of tire size calculations by hand in recent weeks,
but, being the lazy person I am I put up a web page to do the work for me.
If you are interested see:

http://www.volkswagen.org/tiresize.htm

-- David Marshall, Quesnel BC, mailto:david@volkswagen.org --
   -- 78 1.8L VW Rabbit, 80 2.0L VW Caddy, 87 Audi 5KQ  --
   -- 85 1.8L VW Cabrio, 88 1.6L VW Syncro Double Cab   --
   -- Volkswagen Homepage http://www.volkswagen.org     --
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