Stock rear differential ratio is 4.10:1, but there is an alternative 4.3:1 ratio available.  The "shorter" rear end ratio will give you better acceleration with a slight penalty in maximum top speed.  The 4.3:1 gear set is available from Mazdaspeed, Mazdatrix, or any number of RX-7 vendors.  Prices are around $800 for the gear set.

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