With a 6.5T motor you probably want about a 10.5 overall gear ratio. 96/17 gives you 11.5 and 87/21 gives you 8.5.
One a little high the other a little low. Of course driving style and circuit will make a small difference. With an 87 spur I would have gone for a 17 pinion and for a 96 spur an 18 or 19 pinion.
James
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