and I'm not talking about the one's I'm wearing

Alrighty...
I have a 2.0L 5speed running 4.88's
I'm presently turning 245/75/16s, which should almost equate 31x9.50's

I know for fact going to this size, throws my speedo off by 10% give or take a percent
So far this size is "okay" for city driving and I'm getting really really good highway gas mileage as I run 32psi  and my 5th gear still can churn them at a low rpm (2500 is running 100k (60ishmiles)

So as I do want to run taller tires, such as: 235/85/16's or go to a skinny 32" on a 15" rim
I will likely be moving onto a set of 5.12's w/ trackkick 3rds

Thing is... how will this affect the speedo? Is it calculated by the pinion speed or by the wheel speed? I know for fact that I can spin my rear tires mad in the winter, and my speedo doesn't budge
so I'm thinking that the speedo calculations are done based upon wheel rotation
of which gearing should make absolutely no difference

the only difference should technically be more torque, as the engine speed will "more" properly match the wheel diameter

The speedo is driven by a gear in the Transfer Case.  When you increase your tyre size, the speedo reading for a given real speed will be affected, because the speedo reading is proportional to the prop shaft revs, not the wheel revs.

The change in percentage terms is: (1 - New diameter/Old diameter) x 100.

So moving from 31" to 32" will give an extra error of 3.2%

You will also get a 3.2% apparent power reduction (the torque from the engine will not alter, but the lower wheel rpm reduces the power achieved from that torque)

You can also use the above formula to calculate the changes when fitting different final drives in the differentials - just substitute ratio for diameter.

So, to restore your speedo to factory value, you want (remember the speedo error is negative, i.e. -13.2%):

4.88 x (1 + 13.2/100) = 5.52

here is an awsome site to figure out exactly what speed your doing, tires and gears both will throw your speedo off if your trying to match gears to tire size this is the best way to figure it out, is the gen 2 a mech speedo? or is it a computer? if its just mech then all you need is the proper replacement  speedo gears in the tail shaft of the tranny to compensate for your tires and gears, if its computer its as simple as a reprogrammer eaither way this will let you figure out where your at

http://www.f-body.org/gears/

i used it for my trans am as well 2.75 stock gears to 4.10's and 215 tires to 275... using this i got my speedo now within 1mph

The 99+ Vits/Trackers are electrically driven speedos.  You have to get something that changes the signal to change the speedo.  I've got a Yellow Box sitting in the garage waiting to be installed in the XL7 for this reason.

Unfortunately the gear driven, speedo cabled earlier models don't have a good option for correcting the speedo.  You'd think it would be like GM and Ford where you can get different tooth count gears and swap them out but no one seems to know anything about that or have done anything with it.