Leave the Cummins out of this. Mine behaves the same as yours.


Now, back to gassers...
My hot rod has turbos that most people consider too small for a 440. They will light up and start making boost pretty low. Running 65 mph means pulling 2500 rpm. It does that with 18" of vacuum at 13% throttle according to my EFI datalogs. A slight bit of throttle (opening to 20%) to go around a truck will build 2 to 3 psi boost. At 75 mph the engine is turning about 2800 rpm and is pulling nicely for such a low throttle input.
At cruise, I don't see any boost until I'm over 90 mph. At that point the engine is turning 3500 rpm and is starting to work. It is probably the load on the engine that makes the boost. The engine will hold 3500 rpm without boost in 2nd gear.

I don't remember my 85 or 86 Shelby Chargers running in boost at a cruise unless I was treating the speed limit as a paltry suggestion.