That's my understanding too David.
To make 18 inch wheels Mr. Fiat wanted a minimum quantity, I forget the number, but I couldn't get anyone else interested. I never inquired about ordering from Roin directly, if Mr. Roin had a quantity limit to do 18 inch.
The "other" problem with 18 inch wheels is the availability of 285/40ZR18 tires is very limited.
Paired with 225/40ZR18 front there are two sets on Tire Rack.
Paired with 245/40ZR17 front there is only one set on Tire Rack.
The 275/40ZR18 is 26.66 tall … which is OK but not as "good" as the 285/40ZR18 which is 26.97 tall. I've also found owners resistant to the 275 tire because its width is "too narrow". However 275/40ZR18 has far better availability:
Paired with 225/40ZR18 front there are 23 sets on Tire Rack.
Paired with 245/40ZR17 front there are 18 set on Tire Rack.
Dr. Innocenti … I know you prefer wheels of the same diameter, what would you think of a tire set composed of 225/40ZR18 front and 275/40ZR18 rear? The difference in tire diameters front to back with that tire set is only 1.58 inches, a little shy of my usual limit.
I actually like the other tire set better. 245/40ZR17 front and 275/40ZR18 rear. The difference in tire diameters front to back with that tire set is 1.95 inches. With 18 tire sets available on Tire Rack! The 245mm front tire will give better grip during braking. I know the different diameter wheels front to back are not to your liking … but the 17" wheels would not be a special order from Roin.