Ring and pinion options for the ND
 

Having only driven the 2.0 so far all I can say is that the 1.5 could benefit a lot from a shorter diff gear ratio. Having the same transmission and diff ratios with the 2 liter model, it lacks the punch it can have.
Using FM gear calculator, the 1.5 at 7500 rpm red line has a theoretical speed in 6th gear of 304+ kph!!!https://forum.miata.net/vb/images/smilies/eek.gifhttps://forum.miata.net/vb/images/smilies/eek.gifhttps://forum.miata.net/vb/images/smilies/eek.gifhttps://forum.miata.net/vb/images/smilies/eek.gif
I believe a 3.6:1 would suit the 1.5 perfectly and it would make it a more fun to drive, rev happy sports car. I strongly believe that a shorter FGR will make both models (1.5 and 2.0) more track worthy.
Let's wait and see what the tuning industry will come up with.

I wonder what is the compatibility of the first two gens with the ND is regarding ring and pinion options.
Does anyone know if the 3.636 or the 3.909 from the NB six speed can be fitted to the ND's diff casing?

No.

The numbers I've seen show the automatics have a 3.454 final drive (vs 2.866 in manual). No idea if it would work, but I think that'd be the most logical thing to try first and would certainly shorten up the gearing.

I believe the NC diffs are mostly the same. Have you looked into all of those ratios?

My understanding is that the diff on the ND is a new/unique design when compared to other generation Miatas. IIRC it is smaller than the others to reduce the weight. So you could use the AT final drive to liven things up or find

Im not totally sure but I know the ND shares more with the NC than prior.

I also hate to quote a different forum but: https://forum.miata.net/vb/showthread.php?t=628718.

reverse situation and might take a tiny modification. looks like the pinions are different lengths but the gears are the same?

Reading the thread it sounds like the NC pinion is too short for the ND case. Sounds like the poster was going to modify his NC case to fit the larger ND pinion but it wasn't the best route.

Fiat 124's and automatic ND's have a 3.4 rear ratio. That is your best best and pretty close to what you wanted in the first place.