I have used 140/80 tyres on the rear.

You cannot go any wider than this without clearance issues, plus a 140 is at the maximum limit for the stock 2.75 inch rim.

TBH, even a 140 pulls itself into a slightly odd oval shape when squeezed onto the stock rim. Personally I think, having tried both 130 and 140 TKCs back to back, the bike handles nicer on the road and cuts into the dirt better on a 130.

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