
but why will they let you only put on 15" rims ?
In Italy, you can only put wheel dimensions that are given by the manufacturer and written on the car registration document. To put bigger/smaller wheels, you have to present a request to the manufacturer which, in this case, is obviously Fiat, who will (in case of approval) send you a letter to present at the Vehicle Registration Office, which will finally write the new size on your registration document. Sadly, Fiat has quite a name for refusing all requests from cars that have 10 or more years, namely the Fiat Coupé. It is very rare to see 17" wheel Coupés on our roads sadly, some of them are mounted illegally.
The previous owner of my car however had an approval from Fiat: These are 205/50/15 and were added after a request in 2002, when the car was only 4. Normally, I could've had 195/55/15 only. I consider myself lucky

Of course, 16" would look great (I love 17" too), but I have to stick with 15"

Well, at least I saved some money, and I'm happy with the outcome!