Yes 30mm is correct or 1.8 inches more.
0.39 for ever 10mm, if you wanted to go wider on the next set, no problem. I had the same tire size as you except in 21 inch wheels. I wanted Mech P4S only, nothing else. I just purchased the new tires, when I decided if I could go up 10mm wider for my OEM wheel, 0.39 more wasn't much and my current wheel handled it just fine, only .39 more was a no brainer. I'm doing Michelin Pilot Sport 4S 265/40/21 front and 295/35/21 rear and loving it. The cost was 2K. Not cheap but the price for 10mm less wasn't much to nothing and the P4S was harder to find in the 255 & 285 also? I wanted the tire size to match the Vossen wheel I'm getting so did my research on what final size tires I would go with.
I really wanted to upgrade my wheels to Vossen H5 with the P4S to 285 front and 315 rear but as I found out they are not in stock much and cost goes way up for just 1.8 inches more, not realistic and not really worth it to me, so finally it's 265 & 295 is as far as I'm going on tire size, now I'll find the right wheel width for the tires and be done with it.
I added this extra information if your considering the Mech P4S In the future.
0.39 for ever 10mm, if you wanted to go wider on the next set, no problem. I had the same tire size as you except in 21 inch wheels. I wanted Mech P4S only, nothing else. I just purchased the new tires, when I decided if I could go up 10mm wider for my OEM wheel, 0.39 more wasn't much and my current wheel handled it just fine, only .39 more was a no brainer. I'm doing Michelin Pilot Sport 4S 265/40/21 front and 295/35/21 rear and loving it. The cost was 2K. Not cheap but the price for 10mm less wasn't much to nothing and the P4S was harder to find in the 255 & 285 also? I wanted the tire size to match the Vossen wheel I'm getting so did my research on what final size tires I would go with.
I really wanted to upgrade my wheels to Vossen H5 with the P4S to 285 front and 315 rear but as I found out they are not in stock much and cost goes way up for just 1.8 inches more, not realistic and not really worth it to me, so finally it's 265 & 295 is as far as I'm going on tire size, now I'll find the right wheel width for the tires and be done with it.
I added this extra information if your considering the Mech P4S In the future.
