Go with the 31's the 3.90 rear will compensate and the 31's are a lot wider. Both are a popular size so the prices won't be too bad the 31's will cost a little more. For a good highway and good off road tire look for either a BFG All Terrain KO or Cooper Discovorer ATR or Discovorer LT. The LT is being discontinued but their are still a lot of them floating around. If you want a high tracktion but still cheap then a serios off-road tire look at the Cooper STT's they are brand new

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