I'm getting a bit tired of staring at the main hoop of the roll bar in my Miata when I look in my rear view, and then it occurred to me that if I lower the rear view mirror to just above dash level, that should solve the problem. I'm currently looking at mirrors from SPA Design, and they have dash-mount rearviews in either a flat lens or a convex lens, with either a CF or a nylon housing. my question is this: which lens is better?
Convex lets you see more, but you can't figure out exactly where things are in them as well due to the altered focal distance.
Flat lets you see where thing are accurately, but you've less width.
There is no "better", just "different". If those passenger side mirrors with the warning about "objects in mirror are closer than they appear" weird you out, go for the flat glass. If you like those mirrors because you see more and don't care that things aren't where they appear, go for the convex.
Biggest issue I can see is that the ANGLE you will see out of the back may be usless, sure a conVex would allow you to see more, but if the mirror is below the height of the back seats it wont help, so just be sure that the height fo the mirror is still useful, regardless of which you choose, I would THINK a flat mirror 4-6 inches lower would address your problem adequately, I would be cautious of lowering it too much...
Before I put the topper on my truck, I used a convex mirror. It takes a couple days to get used to, but it was the right application. The side mirrors left major blind spots but the convex mirror covered them.
Its no different than a side mirror being convex. If its what you want, do it. A change will take some time to get used to.
The other option is to get a flat one and then go to Pep Boys and buy one of the clip-on convex rear-views. That way you at least have a non-permanent choice of either one.
What year Miata? If its an early one then you can remove the mirror screwed to the windshield header and glue a later one to the windscreen-just a little lower than stock. If yours is glued on you can move it but DO NOT try to pry off the original mirror mount or you'll break the glass. While you're upgrading anyway you can look for a GM mirror with map lights. Cheap and plentiful at junk yards and you'll get a bigger mirror with some extra lighting to boot.
i got so used to using the Wallyworld clip on convex mirror on my 95 with a bar, that I bought one for the 90 without the bar. With the top down, you can see from basically door seam to door seam.
A concave mirror would make you faster. 
My solution was a sliding mirror that could be positioned correctly (hotlinked of course)
oh! I like that.. got a link?
Sure, I bought mine a couple years back from East Coast for $200 but I think I got the last one, I see that they are still available here at Miata Mania
And before anyone points out the obvious, yes this is not a grassroots solution since I'm sure you could fabricate something similar for a lot less dosh, but that's not my strong suit.
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