Does anyone know which wheels, front or rear, the speedomoter/odometer gets its readings from (assuming that readings are generated this way)?

Often the optimal choices in the tire sizes to maintain speedo/odo when upsizing the wheels are not available or have very limited sizes. For example, when replacing stock 17's on a sedan with stock 18's from a coupe, the coupe's rears (245/45/18) have a slightly larger diameter than the sedan's stock 17's (215/55/17) and the coupe's fronts (225/45/18) have a slightly smaller diameter than the sedan's stock 17's. Will the speedo/odo run faster or slower than the original setup? The optimal tire size to stick close to the originals would be 235/45/18 but choices in this size as you all know are slim and not that great a tire as well.

 

Odometer should be reading off of the drive wheels which in the case of the G are the rears.

Check the miata link for the speed differentials with the wheels:
http://www.miata.net/garage/tirecalc.html

Use the wheel sizes from the wheel stickies and put them in the calculator. They should give an idea of what the speed differentials many of the members here are running.