It's always safer to gear it little bit too long to prevent it overrevving. Usually shorter gearing gives you better lap-times, but also sometimes more problems.
Detonation is the worst kind of problem that can happen. It needs a too lean setting of main needle together with too short gearing.
For my mugen 17/21 and 49/46 is always safe for 70m back-straight. There should not be too much difference with Xray.
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