On Mon, 25 Jun 2018 18:06:17 +0100, The Natural Philosopher
wrote:

snip

But if you
mean te circumferenmce divided by 2 PI then that will only change with
pressure, as the belts strech, Not with loading

Nope, also changes *because* of loading.

Otherwise p[eole would be travvling 30% slower when their tyres were
flat,

That would be advised, or possibly 100% slower!

for the same speedo reading

Well, probably not 30% but slower for sure.

Higher pressures will (also 'of course') change the *profile* of the
tyre (as can be seen by the tyre wear) and so the effective
circumference (and the actual (unloaded) circumference).

Not because of any steel belts *stretching* but because the tyre
changes shape (from wide and small to narrow and tall) ... hey, we
could call it 'pantographing'. ;-)

The outcome is similar to a racing motorbike where when the bike is
upright the driven wheel is maximum diameter but when leant over the
diameter changes (smaller) and to the revs would go up for the same
road speed. Same tyre, different results under different
circumstances.

I wonder if Turnip will ever get it and how he will get out of the
position he is currently in? Should be fun ... ;-)

Cheers, T i m