I expect the answer will be yes but just want to double check I can fit a 130/90 onto the rear of my XTR?

Yes!

They are a taller profile so will raise the gearing slightly.

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Thats brilliant, cheers for that.

Are we talking the same as a rear tooth or not even that?

ive run a few figures and if I'm working it out right in theory it works out the same as -1.5 teeth on the rear does that sound right?

The rough increase in profile height is +13mm, but remember that is +26mm in terms of the whole diameter of the wheel. I think you must have worked with 13mm in your calculations which is way you came up with less 1.5 teeth on the rear.

The circumference of a 130/80x17 is 1988mm and a 130/90 is 2069mm, which is +9.6% increase in the final drive ratio. This is approximately equivalent 10 fitting a 15/42 (or 16/45) sprocket combination.

You will need to drop a tooth on the front, or add three to the back to maintain the same final drive ratio that you had before with a 130/80.

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So it's a case of weaighing up if the tyres available are worth the work? i originally looked at it to be able to fit a MT21 rear, but it hardly seems worth it with the choices available in the standard 130/80.