Do Lower Profile Tires affect Speedometer Reading

[George Selby]

At 01:57 AM 11/10/2005, you wrote:
>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.  Let's say my large truck has 31x10.50R15 tires.  If I put on a set 
of 31x12.50 tires at the same air pressure, the PSI between the tire and 
the ground will be less, and the tires will be more likely to spin.  That's 
why big wide truck tires spin out in snow and ice - less PSI between ground 
and tire.

What holds up the car is the total VOLUME of air in the tire.  If you get a 
larger tire (volume wise), you can run lower pressures.  If you get a 
smaller tire, you will have to run higher pressure.  In a typical wheel 
swap volume is reduced because wheel size goes up - the result - you need 
to run higher pressures in your tires.

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

I can assure you that higher profile tires affect the speedo reading.  The 
PO put on 205/70R15's on my 200t - claimed he was scraping on his driveway, 
and needed the extra height.  I say he cheaped out, because the tires 
aren't even 'H' speed rated.  Anyway, GPS based, when my speedo reads 60 
I'm going 65.  Quite the opposite from normal German speedos which 
typically read a higher speed than you are actually going.  Yes, I intend 
on putting some proper size and speed rating tires on the vehicle - I can 
tell handling is suffering both from the lack of speed rating and the tall 
sidewall.


George Selby