Little over 10 inches wide. Should be good on a 8' wheel.
Just so you know 255 is the tread width in milimeters. There is 25.4 milimeters in an inch so all you have to do is devide the 255 buy 25.4 to fid out how wide it is.
With the right BS that will fit the rear real good. To wide for the front. Don't thin you can go much wider than 235 on the front. Not to sure some one will chime in who knows.
This link will tell you what size they are in diameter if you fill in the blanks. http://www.miata.net/garage/tirecalc.html
So, my tyre size of 235/60R15 is 26.1" diameter.
My OE tyre was 205/65R15...so it was 25.5".