17" to 18" wheel
#1
#3
the only time the speedo will be affected is when the diameter of the tire (not the wheel) is affected. a 31" tire on a 17" rim is the same as a 31" tire on a 18" rim. but if the tire on the 17" is, lets say, a 31" tire and the tire on the 18" rim is a 32" tire, your speedometer will be slightly slow, but nothing major. If you were going from a 31 to a 33 or 35, then you might run into your speedo being noticeably slower.
to figure out the change, check this page out: http://www.miata.net/garage/tirecalc.html
to figure out the change, check this page out: http://www.miata.net/garage/tirecalc.html
#4
#5
according to? the wheel size does not matter... its the tire size. if the 18" wheels have tires that measure the same diameter (or within a half an inch) as the tires on your 17", you'd have no issues.