Do Lower Profile Tires affect Speedometer Reading

[Mark R]

Here's the tire calculator I always use:
http://www.miata.net/garage/tirecalc.html
 This will explain the differences in size variations. Again, different
manufactrurers have slightly different actual measurements, but using the
theoretical numbers is close and "precise enough."
 The reason you go to a narrower tire in the winter is actually quite
simple. PSI. Yes, pounds per square inch. The weight of the vehicle (or the
transferred weight if the body is in motion) divided by the contact patch of
the tire. A narrower tire has a smaller contact patch and therefore the
edges on a snow tire (siping and tread) have more force to "bite" into the
surface. Assuming same rim diameter, when you go narrower, the caluclator
will clearly show you also need to go to a taller profile to maintain the
same or similar circumference. Hence the common reference that "You need a
taller tire in the winter."
 Going narrower has some trade-offs. Usually less weight capacity rating of
the tire (not really an issue for most) and less lateral grip on dry
pavement. I go narrower by 10 mm (or occasionally 20 mm when possible), but
don't go for a 135 width tire on an S4 sedan! =)
 Best to all,
Mark Rosenkrantz