It was a last minute decision. My 255s were worn out (like that when I put them on) and I needed to get new tires. The tire guy asked me if I wanted to stick with the 255s or switch to the 245s, which were listed as the correct size for a Z3. Since I was buying tires anyway, I decided to go with the correct size.
If my 255s had not been worn out, I would not have bothered replacing them.
'01 BMW 330Ci
'04 Volvo C70
'87 VW Cabriolet