If your Miata came with an open differential, swapping in a limited-slip is one of the best handling and traction upgrades available, and on NA/NB the answer is a Torsen from a 1994-2005 car. It puts power to the loaded outside tire instead of spinning the inside one.
An open diff sends torque to the tire with the least grip, so a Miata lifts and spins the inside rear on corner exit. An LSD fixes that
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