Do Lower Profile Tires affect Speedometer Reading

[Mark R]

 Yes, a narrower (width) tire has a smaller contact patch at the same
internal air pressure and weight. Of course, this presumes the same or
similar tire carcass characteristics.
Change the weight or air pressure, the contact patch will change. That's
true of all pneumatic springs (tires).
 It's worth noting that tire pressure has a large function on the tire's
weight rating. This is for another discussion, but the current TPMS systems
allow (by statue) a 25% deflation before the warning must illuminate. From
TIA's web site:
 "As an example, a car with a GAWR of 1885 lbs on the front axle has a
recommended cold inflation pressure of 32 psi, which can support 1058 lbs
per tire. If the 25-percent threshold is used, the resulting air pressure is
24 psi, so the carrying capacity of the tire will be less than 900 lbs per
tire thereby creating a potentially overloaded assembly. As soon as the
inflation pressure drops below 26 psi, the tire can no longer support the
GAWR, or the maximum weight on the front axle. If the new proposed threshold
of 25 percent is adopted, the driver could be driving on an overloaded tire
for a significant period of time. This is not in line with the Congressional
intent to keep our nation's drivers safe."
 Personally, I've installed several Smartire systems. In fact, I'm putting
one of my car trailer for next season. I like to be able to read the ACTUAL
tire pressure value and not just a warning light which only has to go on
after 20 minutes of driving while being 25% or more under inflated.
 Tire pressures are crucial... but for the purpose of my discussion, I
presumed the same tire carcass characteristics and the same pressure.
 Best,
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.
>
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