If it has disc brakes, it must be a 15 inch tire. A 225 or 235 75 ratio will doo. If is has four wheel drum brakes, a 14 inch tire was standard. a 225 or 235 75 will do.
The rims were original 14 inch, but with factory front disc brakes 15 inch
rims were 14 inch
The standard tire size for a '66 GTO is 7.75x14.
small
No.
165R15.
Check the sticker on the drivers door jamb. The original tire size should be listed there.
225/65r16
235/75/15 is the size on my 1966 f100
165R15
215 50R-17
No, it is the "donut" type.