I've run an IRC TR8 (4.00 x 18) road legal enduro rear tyre for the last few months to good effect, covered ~2000km including 700k of fast motorway trip and the tyre isn't squared off and hardly warn. Off-road, the rest of the mileage, it's proved decent on rocks/shale and in sand & mud - in fact I've not really got stuck anywhere.

For anyone who runs an 18" rear rim and wants a better offroad tyre than a Conti Twinduro (I have worn out a number of these when running a 17" rim) that's going to last, then I'd recommend the IRC. Cheap too, in the UK only ~GBP40 [€60].

Cheers
Bish