I found that the 315-30-18 Rosso tires had worse performance than the 295-35-18 "Yellows" that came on the car.
The tire size is too wide for max performance on a 10" wide wheel.
Will it go on the wheel?
Yes.
Will you be able to drive down the road just fine?
Yes.
Will it rub on heavy compression dips?
Yes, about the same as the 295s did on the inside portion of where the rear bumper surround meets up at the top with the inner fender.
At maximum lateral load I found that the wheel "moved" inside the tire due to being too wide for the wheel. This is why I am going back to the 295 sectional width. I am also looking into a 30 series vs a 35 series to help with lateral grip.
As there are now more cars in the garage that do the same thing the Speedster does, I find myself driving it less than before, and am now not concered with buying a tire with an @ 6,000 mile life expectancy, thus my consideration of an ultimate performance DOT tire like the P-Zero Corsa.
I can't find the picts I had with them on the car at this time.