Measure...
...across bolt circle holes.
One edge to same edge of opposite hole.
Easier to do on hub side.
But I'll save you the effort by saying Alfa Spider wheels are 108mm bolt circle.
The only alfa wheels with 98mm bolt circle are from Alfetta.
Note: Alfetta stock backspace appears to be too much
for Fiat.
But then, stock Alfetta wheels aren't the attraction.
Finding an Alfetta with aftermarket Daytona Cromadoras
or BWA SportStars is holy grail of wheel finds.
Speaking of holy grail wheel finds, here's a good story.
My buddy recently spied a Fiat Spider with Daytonas on a used car lot.
Being a crafty sort, he kindly pointed out to the dealer
those were wrong wheels for the Spider.
But my buddy, being a generous type of person,
offered to swap his Iron Cross wheels for the Daytonas.
Even threw in some coin to make it attractive for the dealer.
I had tried to work same deal with previous owner of the same Spider.
Considered purchasing Spider to hijack wheels, then flip the car,
but didn't have time/space to mess with it.
Another wheel find story...
Many years ago, I saw a sweet young thing driving an X1/9.
Idly curious and on a summertime random cruise, I followed her.
Nothing purient in mind, simply a way I chase classic cars.
(Also have 3 old ladies with '60s era classics on my radar).
Anyway, as I passed by where the driven X was parked,
I noticed another X parked in tall weeds and obscured by a boat.
Nothing special appeared, until I drove past and looked in rearview mirror.
Barely visible was what looked like it mighta been a Daytona.
Heart beating, I circled around for better look.
In fact, it was a single Daytona on a derelict X.
Not wanting to stop while wearing slacks/dress shirt
and driving Twin Turbo ZX, I went home & changed to grubbies
and grabbed my X1/9.
Knocked on the door to inquire about the Daytona wheel.
Turns out, the girl's dad had a set of Daytonas, and a set of CD30s!
They soon became mine...
Point of stories (besides for amusement) is that
wheels are where you find 'em if you turn over enough rocks.