My Regency has the Cummins 6CTA8.3-300 engine with the Allison MD3060 6-speed transmission. I use the cruise control when on the freeway. I set it at 65 mph. At that speed the engine is turning at about 1500 rpm while in 6th gear. When using the transmission to control descent on a hill I have seen the engine speed go to somewhere in the low to mid 2000 range for a short time period. Currently I am working on adjusting the Pacbrake and Pacbrake has told me that the back pressure should be 65psi at 2800 rpm. I am very hesitant to take the engine to 2800 rpm.

Does anyone know for certain if my engine can sustain 2800 rpm for any amount of time?

On my 5.9L it has a governed limit of 2800 RPM. I will run it to that limit down a hill in any gear.

On yours just put the trans in any low gear, probably 3rd is best and floor it. If traffic permits and on the flat and level, hold it floored until the engine stabilizes at the highest RPM, that will be "the top of the governor". That would be the safest speed you can run your engine up to going down hill. I would bet it probably is around 2800 ROM also.