By chance I just did something very similar, and the results are like yours
that is somewhere between the two theoretical possibles. The circumference
of my tyre is 1940mm regardless of whether it is unladen at 24psi, laden at
24psi, or unladen at 10psi. The distance travelled on the ground between
two points where a mark on the tyre was on the ground one revolution apart
was 1885mm at 24psi and 1876mm at 10psi, or 0.5% less. The rolling radius
(centre of hub to ground) was 290mm at 24psi and 257mm at 10psi, or 11.3%
less. 290mm gives a theoretical circumference of 1823mm and 257mm one of
1615mm. So the actual distance travelled is noticeably greater than the
calculated circumference but noticeably *less* than the actual circumference
of the tyre. I did the 24psi measurements again after the 10psi
measurements and got exactly the same result.
http://www.chris-longhurst.com/carbibles/index.html?menu.html&tyre_bible.html
emphasises the importance of using rolling radius when calculating effects
on speedometer readings (which is where I presume all this started as I
don't recall seeing a question only a response) and shows that it is
measured from the centre of the wheel to the contact patch on the road, at
the recommended inflation pressure and 'normal' loading. His calculator for
my 165/70 14s gives a rolling radius of 293 and a circumference of 1843
which are about 101% of my measured radius and calculated circumference, but
are about 98% of my measured distance travelled and 94% of my measured
circumference! So either there is a fiddle-factor when calculating distance
travelled for each revolution, as opposed to circumference, or all speedos
are designed to be inaccurate!
PaulH.
----- Original Message -----
> I decided to do a little experiment using a wheel off of my MGB and one
off of
> my Bugeye. ...
>
> If anyone else is willing to duplicate this, I would like to see what you
come
> up with. Note that I didn't repeat the whole exercise to see if the
results
> came out the same.
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