Do Lower Profile Tires affect Speedometer Reading

[Henry A Harper III]

> 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

Are you absolutely sure that the contact patch total area is determined
simply by the width of the tire and not at all related to air pressure
(hmm...measured in PSI) in the tire? (Within the limits of the tire carcass
stiffness.) Narrower/different shape contact patch, sure, but actually
smaller without changing air pressure, I don't think so.