Huge (Huge ) gurgled happily, sounding much
like they were saying:

Been trying to understand tyre markings on my trailer tyres. It is
marked as 205 75 R14

That should mean (I think) rim diam is 14", height of tyre is 75mm and
width is 205mm.

Form this you would assume overall diam of Tyre would be 14" (i.e.
355mm) plus 2 x 75mm so overall width of 505mm Yest if I measure it
the overall width is 635mm significantly different.

Anybody know why the discrepancy?

The "75" is the aspect ratio. The lower the number, the lower the
sidewalls.

And is given as a percentage of the nominal tread width.

So, in the OP's example, you've got 355mm + 2x(75% of 205mm) = 355+2x154
= 663mm.

Tyre sizes must drive the metrication fascists bonkers; three numbers,
one metric, one imperial and one dimensionless

Not exactly "dimensionless".

not to mention the speed rating which is an arbitrary index, not in
alphabetical order, to a speed in mph (that's the "R").

No, that's R-for-Radial. There's a piece of information we're not given
in the original post - the load & speed ratings. A decade or more ago,
tyres used to be typically labelled "205/75 HR14" - where the H was the
speed rating. Now they're more typically "205 75 R14 87H" - where the 87
is the load rating and the H the speed rating.

http://www.blackcircles.com/general/sidewall