What Was the Standard Wheel Diameter for '65 Signet Convertible?

I think you mean late 64, early 65. At any rate, my 65's build date was December 4, 1964, so kinda early in the production year. Rules may have changed later.
BTW, related to another post a while back, I note that your center grille is body color, and the inset area of the front valence, under the bumper, is painted black.

No, I bought it late in the year (65) or January, 66. The dealer (I think is now called Dadeland Dodge and Chrysler) had the car for several months after the sale fell through because of the 4 speed and Commando engine. A young guy (rich kid) got it for his graduation from high school (June 65) to drive to college. (UAF) He put a thousand miles or so on it and brought it back and traded it for a slant 6 automatic. He told the dealer he didn't like driving the 4 speed in traffic, it used too much gas and most of all it was too (resonator) loud. The black in the picture must be from the negative, or a shadow. It had no black paint on it. I owned it clear up until 1975, until in a fit of anger, traded it in on a new Chevy step side. (Stupid!) I've searched for it (I have the VIN Number also) for many years without luck.