[Biturbos4] Winter wheels

[John Koenig]

My sentiments exactly.  I remember a rule of thumb (though not its source)
saying that overall diameter/circumference within a few percent is
acceptable (assuming one is trying to match OE size).

  -----Original Message-----
  From: Mark Rosenkrantz [mailto:speedracer.mark at gmail.com]
  Sent: Thursday, November 08, 2012 10:02 AM
  To: Grant Lenahan
  Cc: John Koenig; <biturbos4 at www.audifans.com>
  Subject: Re: [Biturbos4] Winter wheels


  Back to winter tires:
  Assuming identical tires (compound, features, etc.), narrower tires are
better on ice and snow, since PSI onto the surface to "bite" is critical.
This means ice, snow, hardpack.... but not wet or dry pavement.  A wider
tire has more surface area and therefore less PSI onto the surface.  This is
why desert tires (designed to float on top of the sand dunes) are wide.

  The minus one or minus two concept is simple- go with a smaller diameter
rim (the minus) and a narrower width, but taller sidewall tire to match
outside circumference to baseline, which means speedometer will be correct,
etc.

  A site which I've used for years provides the graphics:
  http://www.miata.net/garage/tirecalc.html

  It's worth noting that every manufacturer has slightly different specs...
mostly related to tread block design and height.  In other words, one
manufacturer's 245/50-17 tire isn't exactly the same height or circumference
than another manufacturer's tire, despite being the "same size."  So (very)
close is good enough.  =)