[wilf leek]

I think there is some correlation also between a tyre's speed rating, and it's loading (i.e. vehicle weight).

The speeds given in the table above are probably at the tyre's max loading, (or the tyre manufacturer should publish a table showing how to derate the tyre with increased loading).

Cobras being light in the greater scheme of things, you "may" get away with a lower speed rated tyre. However, your insurance company might well think that a vehicle capable of 150mph + should have tyres rated for that speed.

Using 16" wheels on my own car means I can fit Z rated tyres.