I need to get a new transmission for my turbo Miata. I found JDM 6-speed transmission with less than 50k miles. My questions are: 1) are the gear ratio different than US specs, 2) are JDM transmissions any different.

I should mention I am running around 300whp on my MA Miata.

 

My understanding is all 6 speeds are the same, with the exception that MSM transmission may have a stronger gear set. All I can find on the MSM difference is speculation though.

 

Nobody can (has yet) definitively say otherwise.

If you can find some evidence of this, the internet would like to know.

I have seen a post on here of one stripped (IIRC in a gearbox shop), and nothing was found, FWTW. Different heat treatment and things like that may not be readily identifiable. But given the MSM was just an NB with a piddly turbo slapped on, they would not have seen a need for a significantly stronger box - in fact one can speculate that the design criteria could be stated as 'keep it cheap'. The failure behind the 'circlip mod' suggests nobody looked at the box and said, 'hey, with all this extra power this is a likely failure point', and remember that the failures are coming with around double the power levels of the NB, or more.