Jeff, of course, is correct about the 3 nubs for the "dog dish" style wheel covers. However, if you get a wheel like the 1968 wheel he pictured, it will be incorrect for your 66. The raised center ring where the holes for the wheel studs are (circled in yellow on the first picture attached), only appears on 1968 and later years. The correct type wheel for 1967 and earlier years, is shown in the next picture.
In addition, if you want to be absolutely correct; your wheel should be dated no later than the month of your build date, and could be a month or two earlier. Besides being stamped with the FoMoCo Oval, wheels were stamped with year, month and manufacturer of the wheels. Wheels were supplied most often by the Motor City Wheel Corporation, but also, although not as often, by Kelsey-Hayes. From what I have seen, the K-H wheels were stamped on the inside bead of the wheel, or on the inside part that is covered by the tire. All of the Motor City wheels that I have seen are stamped on one of the center raised portions of the wheel (third picture). Stamping seems to take the most common form as "x"M"z", where "x"= the year (a single number), M = Motor City Wheel Corp, and "z" = the month (a single alpha letter). Thus, in the first example I pictured (from my January 31, 1967 build 67 Convertible), the "7MA" translates to January, 1967. While this is, from what I have seen, the most common configuration of the stamping, I have also observed it with "number - M - alpha" (the fourth and fifth pictures - "8MF? = June 1968). In addition, I have also seen them stamped with the month first and the year last (pictures six and seven ? ?HM4? = August of 1964).
The years are the actuaJ years, not Model years. Thus, if your 66 was built prior to January 1of 1966, the wheels should be stamped with a ?5?
Another thing to be aware of is the width (inside bead to inside bead) of the wheel. All 1966 V8 Mustangs with steel wheels had 5? wide wheels.
Finally, the good news ; MCA Concourse Judging Rules for 1964-1/2 through 1966, note that spare wheels and tires should be ?Proper type and size for Model year?. Wording for the wheels and tires on the car is similar. It doesn?t seem like they go into detail on the date coding. As for the width of the wheel, once a tire is on, it?s pretty hard to tell, unless you have a very large caliper or set of dividers.
Hope this helps clarify things.
Bob