John Carrier wrote:

Early F-4B's were good for an easy 2.2 (skinny wing and lack of add-on
antennas) and I suspect with the right conditions and trimmed engines,
somewhat more. Best I saw was 2.05 out of a late J (S-config without the
slats) and it had the wing pylons attached. The Skyburner F-4 was the early
one with small nose and canopy ... certainly not representative of
production A/C.

Most of the F-4 (E-model) mach logs I saw topped out
about 1.8-1.9, with only a few FCFs pushing it to 2.2
or so.

However, in those (1970) days of slick wings (no slats)
and the short gun fairing, I did see one F-4E come out
of phase and pull a mach 2.4 FCF before returning to
the flightline at Korat. Nobody believed it - so they
checked the TAS system. It was accurate. A dirty old
warbird from the Korat flightline could still pull 2.4
without any preparation (other than pylon/stores removal).