When upgrading your 90-97 Miata to a differential with a different gear ratio, your speedometer will no longer show the correct speed.
 
In extreme cases, it can be quite a difference, for example going from a 3.9 to a 4.6.
 
Maruha Japan has brand new driven gear units as a direct replacement to correct the speed readout on your guage cluster.
 
Custom order from Japan usually with ETA of 2-3 weeks.
 
Available ratios
3.99
4.6