Two friends of mine opinions: One guy has an excavation company, he loves super singles on his dual axle dump trucks... which are mostly run on local roads. Another guy is a long distance trucker - he does not want super singles, mostly due to the costs: initial cost, repair cost, and replacement cost. So I have no idea if this helps anyone!
Also, why would a super single get measurably better fuel mileage than duals? I would just assume that two narrow tires would have less rolling resistance than one big wide tire.