How many 6.5in driver equals one 8in driver

The Math tells us 2.5 is the answer. I would up the ante to (4) 6.5 inch drivers equaling a single 8" driver. When designed and properly enclosed, an 8 inch driver/woofer can sound outstanding!  Same goes for running (2) 8-inch driver/woofers in series.

How did you come up with (4) 6.5in. drivers equal an 8in. driver?  On what basis?

By faster I mean that a smaller woofer inherently must move further to move the equivalent amount of air that a larger one does so to keep up with the signal & remain tight & clean, it must accelerate & decelerate more quickly. This extended range of motion & required speeds necessarily creates more potential distortion all other things being equal. Paul Klipsch was noted saying that that the more motion in a driver, the more distortion. 

By faster I mean that a smaller woofer inherently must move further to move the equivalent amount of air that a larger one does so to keep up with the signal & remain tight & clean

But if you have multiple smaller drivers that equal one large driver, the distance moved is the same for both.

Here is an example of driving four 5in. driver.  Basically it is two in series and then each two in parallel with the other.

It is from Zaph, a very well regarded speaker designer.  You can see below how the drivers are being driven.  The speakers are MMTMM layout.  In this way, you can have an overall impedance of 8ohm.  If you drive them all in paralle, the impedance would be 2ohm which is too low.

The kit is available at Madisound.

https://www.madisoundspeakerstore.com/2.5-way-speaker-kits/zaph%7Caudio-za5.5-tall-tower-2.5-way-pair/

Enclosure drawsing:

https://www.zaphaudio.com/ZA5/ZA5.5tt.pdf

I have yet to hear a speaker w/ multiple small woofers that can match a big box w/ big woofers assuming both are  of good quality. The whole concept of insisting on narrow speakers to minimize diffractions & improved imaging is questionable which is mostly why speakers use multiple small woofers.