Yes, you have the correct understanding in my opinion. Tire pressure has
only a small effect on the tire revs per mile. The so called "load
radius" of the tire can decrease quite a bit as the pressure is lowered
but the effective "circumference" is not reduced by a corresponding
amount because the belts in the tread do not shrink nearly as much.

I have actually measured the effect and found it to be a minimal.

Mark

Jack wrote:
>
> re:  "All dependent on tire inflation pressure and vehicle mass. Lower
> pressure means greater indentation (smaller radius) at contact patch,
> therefore higher RPM."
>
> Ok.  I need some help understanding this.  Isn't the radial tire tread
> similar to a tank track in that its length doesn't change no matter how
> heavily loaded?  (If it matters, assume steel belted.)  And that tread
> must still travel its full length in exactly one revolution.  So what
> difference does the variable radius have on RPM?  What am I missing?
>
> //Jack
>
> On 6 Oct 2002, at 12:08, Andrew Grebneff wrote:
>
> > >If you attempt to do these calculations by reference to
> > >circumference you will introduce unavoidable errors into your
> > >calculations because circumference bears only a loose correlation to
> > >revolutions per mile conditions for any given tire. Revolutions per
> > >mile is the only thing that matters as far as your gears are
> > >concerned. For accuracy, it is best to use manufacturers' stated
> > >revolutions per mile at X MPH for each tire and IGNORE both tire
> > >height and IGNORE tire circumference information. A revolutions per
> > >mile number that matches what one would get by calculating that
> > >number based on either tire height or circumference is almost
> > >certainly not accurate.
> >
> > All dependent on tire inflation pressure and vehicle mass. Lower
> > pressure means greater indentation (smaller radius) at contact patch,
> > therefore higher RPM.
> > --
> > Andrew Grebneff
> > 165 Evans St, Dunedin 9001, New Zealand
> > <andrew.grebneff@stonebow.otago.ac.nz>
> > Seashell, Macintosh, VW/Toyota van nut