Do Lower Profile Tires affect Speedometer Reading

[Huw Powell]

>>At the same inflation pressure and weight, any tire will have the same
>>area on the road.
> 
> 
> At the same inflation pressure and weight, a tire with a larger contact 
> patch will have a lighter footprint than a car with smaller contact 
> patch. 

sorry...

> How pressure relates to contact patch size - the more air in the tire, the 
> higher the pressure and the smaller the contact patch.

Exactly.  The PSI in the tire *exactly* corresponds to the pounds 
(weight of the vehicle) per square inch (contact patch size).  For the 
sake of simplicity we are thinking four identical tires at identical 
pressures, if they vary, the patches will be distributed according to 
simple algrebra.

I used to be skeptical, and then I saw the light.  It's physics, pure 
and simple.

But remember, different tread patterns yield very different natures of 
contact patch.  IE, a lick will "sit" differently than a big knobby mud 
and snow tread, to get the same "contact patch"

> You start to learn this stuff when you mess with big tires (like 
> 35x12.50R15's)  I run less than 20 PSI in my Jeep, under 5 PSI to drive on 
> the sand.  If I put stock size tires back it I need like 30 PSI, 15 lbs on 
> the sand.

You run 5 psi to get a huge contact patch.  "Stock" tires would of 
course be awful at that pressure, but you're running them on paved 
roads.  And of course, sand creates a weird situation, oozing and 
flowing and letting you sink.  But the physics still holds.

-- 
Huw Powell

http://www.humanspeakers.com/audi

http://www.humanthoughts.org/