Will running a 40 series front tire (my car is equipped with 45 series 18"s) throw off my speedo and my odometer? Thanks!

It depends upon the tire width. the "40" or "45" is a ratio, rather than a measurement. So theoretically a 40 could be taller than a 45. What is your stock tire size? And what size tire are you thinking about using? You can compare and determine speedometer effects of varying tire sizes here:

http://www.miata.net/garage/tirecalc.html

-T