as the post says what are the best tyre available at the mo for an 18inch rear wheel?
or am i better of getting a 17inch wheel from an nc29??
cheers.

Take your pick, Dunlop Alpha10 or Bridgestone BT090.
Both as good a tyre as you can get, both available in 18", why anyone worries about running an 18" rim I have no idea.
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A nice feature of an 18 inch rear is the way that you really get a noticeable change in rpm on a steady throttle from upright to cranked over to back up again. It's nowhere near as noticeable with a 17 and It always makes me smile.

I've got the Alpha, but of course the bike isn't up and running. By all the reviews they should be awesome.

Alpha10s are V good and sticky. On the road though they square off very quickly indeed so not the best bit of 'everyday' rubber. I've not yet managed to get 2000 miles from a rear a10. If it's purely for a road bike (and it's used fairly regularly and in all weathers) I'd personally think it better to go for something a bit less soft and sporty really. BT92s perhaps?