Here is a nice tool you can play with to find a wider tire that wont upset your speedo.You could switch to 225-60.15 for 1/2 inch wider or 225-60-16 for 1 inch wider.Read the information below this tool as you change tire sizes for speedo changes to see the different. Try this tool and judge for your self
http://www.miata.net/garage/tirecalc.html