I am contemplating switching to Miata gears in my FC. I am aware of the modification per the FAQ on Mazdatrix.

Can you use the Miata 1st-4th and an RX-7 5th gear?

I did some calculations below, please note tire size and 5.12 rear gear. I'd like to keep my 5.12 rear and 225/45 or 225/50 are probably my best tire choices.

Based upon this, it looks like the Miata 1st-4th and RX-7 .756 5th with a 225/50/15 tire would be my best choice? I have hit 150mph on some of our roval tracks, so the Miata 5th just won't work with the tire choices or 5.12. What do you think???? My power curve is below, HP and torque intentionally removed

 

Some Miata guys use NA FC 5th gears in their transmissions, but it's a mix and match type of thing, so you won't necessarily get the 0.754 ratio. Check www.miata.net for info.