This should give you the info your looking for
97-03 gp's
Q: What is the factory size?
A: 225/60/16 on gt/gtp's
Q: What size is the factory rim (GT and GTP)?
A: 16"x6.5"
Q: What is the weight of the factory rim (GT and GTP)?
A: torque star = 14-15lbs
A: crosslace = 15lbs? (not 100% sure)
A: flat 5 star = 17lbs-18lbs? (not 100% sure)
A: snow flake (3 star) = 21-22lbs
Q: What is the factory rim offsett (GT and GTP)?
A: +52mm
Q: What is the factory rim bolt pattern (GT and GTP)?
A: 5x115mm
Q: What is the factory rim bore size (GT and GTP)?
A: 70.30mm (per RI.SHO)
Q: What size will work ?
A: TRY ME OUT
http://www.miata.net/garage/tirecalc.html
or
http://www.1010tires.com/
Q: Will 245/50/16 fit on a factory rim?
A: YES, but not safely, in emergancy manuvers the bead could roll and the tire could blow out, WHICH IS A BAD THING