245/45/R17 is quite a bit bigger than stock size, and too wide for our cars. For a 7" wide rim 205/45/R17, 215/40/R17, and 215/45/R17 are good sizes. The first being almost identical to stock rolling diameter, second a bit smaller but wider, third is wider but also but with a bigger rolling diameter. You can go with 225/45/R17, but then it starts getting even more over stock rolling diameter. On a 7.5" wide rim all those sizes work fine, they just support the sidewalls better. Try this site, http://www.miata.net/garage/tirecalc.html
, to see just how much it will throw off your speedometer. Just put 205/50/16 in as the original and whatever new size you want to compare.