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         A 140 will fit - I suppose it depends on the type of off road riding you mean, wet boggy, gravel or a mix - this I would say affects your choice.
          Say a Karoo for the wetter stuff v say a K60 Scout for the drier stuff.
          A lot comes down to trial and error - tyres / mile is also something that you will only get a feel for by experience.
          Hope some help. 
         
        
        
        
        
        
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