Tire Size

[Mike98'240]

215/45/16 in the front~
225/45/16 in the back~

or 215/50/16
or 225/50/16

[AKADriver]

How about 225/50R16 all around?  Running a 'stagger' like that will only promote understeer.  Unless you desperately need the extra clearance of a 215mm tire, I don't see a reason to use it.

What width and offset are your wheels?

[uiuc240]

...and that's not even enough stagger to matter.  if you have tons of power and need a stagger, it'll be something like 225 in front and 255 or 265 in the back.  215 and 225 is a negligable difference.  just stick with 225's all around and that way you can at least rotate the tires front to back as they wear.

Eric

[Tekryder]

I dont know if this will help, but my tires are 195/50/16 in back and 205/45/16 in front and my speedo is between 5-10 mph too fast.  

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

[240 2NR]

What wheels are you putting these on?

Which chassis?  S13 or S14?

S14 stock tire size is 205/55/16 (iirc).  S13 stock is either 195/60/15 or 205/60/15.  

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

This should help you get a baseline of general tire sizes and specs, though check with the real tire specs for revolutions per mile to assure the proper rolling diameter as tires are not all exactly the same as the standard sizing would lead you to believe.  Some tires, like Kuhmos, tend to run tall, others may tend to run wide.  

I also agree about the lack of substantial stagger the extra 10mm will provide, and think it would be better to go the same all around to allow for tire rotation.