The factory size for the 1965 GTO is: 205/75-14
The standard tire size for a '66 GTO is 7.75x14.
P-Metric size is P215-70 R14.
The standard redline tires were G78 x 14.
"64 and '65 were the same 14 x 6 wheels with Rally wheels as an option in '65. Tire size did change from 7.50 x 14 to 7.75 x 14 for "65. The tire options were the standard white wall and the US Royal red line tires.
Stock tire and wheel size was 6.95 x 14. http://www.automobile-catalog.com/tire/1965/829520/ford_mustang_hardtop_six.html
Stock tire and wheel size was 6.95 x 14. http://www.automobile-catalog.com/tire/1965/829520/ford_mustang_hardtop_six.html
215/75/15
185/75 14
8.15 X 15"
No
Stock wheel size was 13". A 175/80/13 Closely resembles the stock tire size.
G78 14 or a p 215 75 r 14 tire.