Yesterday I mounted up a new Heideneau K60 Scout on the rear of the Dakar. This is the second one (both 140/80-18) in a row for me on the 18" Excel rear wheel I had laced up by Woody's Wheel Works in February. The first one went just over 6,200 miles and still had about 2 mm of the original 11 mm or tread in the center. I probably could've gotten another few hundred out of it on the street, but it was toast for anything off pavement. I was surprised a little that the mileage was as good as it was considering I've run it at 20-24 psi for its entire life.
The front K60 was mounted at the same time as the first rear (~6K ago) and still looks practically new. I run about 18 psi in it:
As long as they're available I think this will be my tire of choice for the Dakar. They are surprisingly good off-road and fantastic on pavement.