An RPM is too low for highway cruise
when 'reversion' of fresh air and fuel mixture begins to happen backwards into the intake port
in the last few degrees before the intake valve closes.

Maybe a fast acting pressure sensor
or a very compact
'hot wire mass flow meter'
could be mounted in one cylinder's intake port somewhere
and watched with a o'scope
to spot the RPM where reversion happens

With some many engine makers running variable valve timing these days,
I would bet that there are engineers at most automakers who could quite closely estimate at what RPM a 440 ci Mopar V8 would begin reversion.

A partially closed throttle and high intake manifold vacuum probably makes reversion happen sooner.

'Steps' machined into the port just above the intake valve seat,
and then additional such steps every half inch or so,
act as 'sonic wave breakers' and reduce reversion.

The 5.2/5.9 Magnum V8s have a designed-in step made by an on-purpose miss-match at the end of the manifold runner at the joint prior to the mouth of Magnum cylinder head intake port.
This is for anti-reversion.