MX5 with RX7 5th Gear Envy (donor needed)
 

Wish I had a RX7 but the 120-mi/day commute I have dictates the MX5's mpg.
That's right, 120-miles a day, 2400 miles per month, 28,000+ miles per year.

I've gone through about 3 cars and 1 motorcycle on this commute.
It covers the length of highway 154 from Santa Maria to Santa Barbara, Ca.

Presently my 1990 MX5 (210,000 miles) just finished a transmission rebuild.
The tranny appear to be the same basic unit (different ratios) as in
'87~'91 RX7s (non-turbo, of course).

I was about to put it in the car again but noticed during some research online
that the 5th gear set on a '87~91 non-turbo RX7 should slip right into
the MX5's transmission to get me about 300 rpm lower on the highway.

Only good, complete RX7 transmissions are for sale usually.
Plus nobody seems to sell 'used' tranny guts.
So, where do the broken case (wrecked) trannys go?

Jeff
1990 MX5 Miata (no, its' not just a chick car, they put V8s in them too),
210,000 miles.
1983 Mitsubishi 2.6 P/U, 240,000 miles.
1995 Honda Pacific Coast M/C, 132,000 miles.
1997 Honda XR250R dualsport M/C, 1,600 miles.
1988 Conquest TSI (Mitsu 2.6 turbo, intercooled, Lim-slip, etc.),
232,000 miles when sold.
1980 Dodge/Mitsu 2.6 PU, 164,000 miles when sold.

n/a trans are cheap enough to buy them used in good condition.

You can find them for $50 or less if you look around a bit.

A fair amount of Mazda race shops have swapped parts from Miatas (particularly gearing) into the RX7 case. Its likely that the opposite can be done as well. I'd start by calling around to Mazda specific race shops in your area (perhaps Mazdatrix). There have been some threads about this topic on this forum's Race section so you might want to use the Search feature to do some research there and perhaps also do some research on the Miata forums.