I think that you will then need to change both tyres given that the old ones are no longer available. I had Diablo Rosso 2 fitted to my XTX and they were good for another 12,000 km more or less as the originals, but I have recently fitted Michelin Pilot 3s I think and so far so good, they SHOULD be a good all weather tyre as well as long lasting.

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(Now sold on, sob) 2011 Blue XT660X with gold wheels, was 26,500 km.
Engine mods: K&N Stage 1 filter, DNA Stage 2 filter, snorkel removed, Kev fuel mod fitted.
Plus: smoked Puig screen, Yam aluminium sump guard, Yamaha / Acerbis handguards, Givi Trekker aluminium side cases, Leo Vince X3 cans plus Kev front fork mod and Fender Xtender. Just fitted Michelin Road Pilot 3 tyres.

SOLD the XTX for a Super Ten 1200. And just bought a Raptor 700 so staying on here for some tips....