Doug replied:

Rob,
If you go to any of the major tyre suppliers they will be able to give you some details. I have/had exploration vehicles fitted with both Bridgestone 10ply radials, Michellin 12ply radials and MRF 14 bias ply tyres. Generally the bias ply tyres generate alot of noise and excessive heat due to heavy lug patterns and wear quite a bit faster than the radials. Am getting quite good wear out of the Bridgestone tyres both on and off the road with the off road work quite tough in the mineral exploration environment.

Tony replied:

Rob, Dunlop Roadgrippers in their various forms had 6,8,10 and 12 ply. Firestone also had 8,10,12 and I think a 14 ply. The firestone were/are a very "chunky" looking tyre and very cheap compared to the Dunlops.

Rob replied:

Thanks for the info,I'm currently running 8ply roadgrippers, but the next set of tyres is due soon and I will probably go for 10+ ply, the road handling characteristics of the tyre is of littttle concern as I do not use it as a daily commuter in suburbia.
Rob