They are not 6.5" or 8", they are somewhere in the middle 7"-ish.
They are a super shallow design where the motor is inverted to reduce mounting depth. BMW has been using this design for years as well.
I installed some earthquake 6.5" subs in place of the stock ones (on a dedicated amp) thinking the extra power handling and excursion would improve the bass but was seriously disappointed.
The stock subs (on the same dedicated amp) have way more bass and actually handle more power then the earthquakes.
It may be possible to fit 8" BMW subs down there in a custom enclosure(tons of work), but don't waste your time and money on 6.5s.
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