195/50/15 , 195/55/15 , 205/50/15 ?
#2
I ran 205/50-15's on some OEM 1999-2000 Miata alloys (15x6) with no problems. The 205/50 is wider, and is actually a bit smaller in diameter than the stock tires, so expect a slight improvement in straight-line performance too. It's also an easy size to find. The 195/55's aren't very common.
#3
I agree with rustychops41. If you're sticking with the OEM 15 x 6" wheel, the 205/50 is the best upgrade and will provide better handling because of its wider tread and lower aspect ratio (less tire squirm on the rim). Also, a 6" wide rim is still an acceptable size for a 205/50 tire.