Wow, many questions.
Plus sizing as it is often called usually compensates the increased wheel diameter by decreasing the profile of the tire. The following link should help you out immensely. Plug in your current tire/wheel sizes. Then change the rim diameter to 15, 16 or 17 and you can play with the sizes to get the cloestet possible tire circumfrence.
http://www.miata.net/garage/tirecalc.html Another excellent site is
www.tirerack.com. They not only offer a wide selection, but outstanding customer suppport and usually the best prices.
The following link will show your '98 SE and Tirerack's available wheels.
http://www.tirerack.com/upgrade_garage/W...E&autoYear=1998 Aaron
