I'm 5' 6.5" with a 30" inseam. With the seat in its lowest position and the back in a fairly upright position I have maybe 1" clearance to the moonroof. I can feel my hair brush against the roof almost constantly.

I like to drive in this position for technical reasons--you want a slight bend in your knee when you have the clutch fully depressed and fully extended both wrists should reach the top of the steering wheel.

I have a feeling I'll have to compromise this driving position in order to wear a helmet during autocross. We'll see. The large rearview mirror I mentioned may also obstruct the right side view.
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Gb500

 

For autocross, where the First Rule of Italian Driving ("what's behind is not important") applies, you should be able to "roll up" the mirror so it's up out of the way against the top of the glass. The technique works fine in the later Miatas, where the mirror is even more intrusive, and since the RX-8 mirror mount is similarly articulated, it should work in the 8 too.