What kind of thread could this be?

[The Nomad]

Why I triped 10TPI when the brain said 20 TPI (or did I mean 1T/mm?)

Oh well put it down to old age and lack of sleep ...

Avpx

[Dave Plowman (News)]

How old is this Land Rover?

[Adrian]

There should still be a steel frame inside the wheel, albeit padded externally.

Try the same as you'd try for a stuck roadwheel. Loosen the nut, go for a drive.

[Bob Minchin]

Also known as 1/4 BSB British Standard Brass, a constant pitch series

26TPI irrespective of the diameter. Most common is 1/2" BSB as found on the lampholder threads for table lamps.

[Chris]

I don't doubt it, but the frame could be made of cast iron for all the good it will do to clump it when its smothered in rubber. If you can't transmit the shock to the splines thanks to all the rubber, there's not much you can do - other than use a puller.

Won't work for a steering wheel; different forces in play.

[Johnny B Good]

To answer the question posed in the Subject line (having viewed all the many replies thus far) would appear to be "Very long." :-)

[polygonum]

Even more remarkably, mostly very on-topic. :-)

[Another John]

Brilliant :-)