Re: Does 12" + (5.25" x 4) = 8"?
No, you'll be absolutely fine. Just be sure to set your crossovers correctly on your headunit or amp, whichever you're using. 5-1/4" mids may not be as commanding in the midbass region as their 6.5" counterparts, but there won't necessarily be a gap. If you're unsatisfied with the amount of midbass you're getting, you can seal and lightly deaden your doors (so it acts more like a sealed enclosure rather than open baffle), you'll be able to extract a lot more midbass out of your mids.
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