You know, I was actually considering getting one of those collapsible donuts until I read kodebuster's comment :
Even if I could fit a donut in the trunk, what would one do with the regular sized wheel that just came off. The passenger seat is not an option if someone is sitting in it.
I hadn't even thought if that. I am not about to leave a $200 wheel and $200 tire on the roadside, so I guess that spare idea is not workable for me and my roadster.
Guess I could always do what BLACKSRT-6 did and put a trailer hitch on along with a trailer designed for a motorcycle and put a 19", an 18" and a spare trailer tire in that.
NOT !!!
( he uses his to carry his autocross wheels/tires to the races, so for him it makes sense )