I posted recently that my Heavy Duty Field & Road 15.5X38 were 140# per tire. My BFG 30 degree with slightly more tread wear was 160# per tire. Both were 6ply. Deep Treads seem to be the heaviest. Plain Field & Road (non all-traction f&r) are pretty light. Armstrongs are a cheaper less durable tire that also may be light.
What would be the lightest model regular bias ply tire out there?