Most definately not too fast to have the mds cut on. I had plenty of times on my trip when mds cut on at 75.

 

Head and tail winds will have a lot to do
with how high a speed the MDS can stay activated,
as will concrete versus blacktop pavement.

I used to not believe the concrete vs blacktop effect
but I-40 from Raleigh to Wilmington is about half
concrete and half blacktop and on the tests I did there
driving back and forth always showed the concrete
sections 1-2 MPG better.
(but I still hate concrete joint jolts)

If you are looking for something to do on a long trip,
play around with your tire air pressure
on different sections of the trip.

Higher tire air pressure will allow the MDS to stay on
at higher speeds.

You can also find the speed just before the MDS will kick out,
then roll down your windows to see the worse aero
make the MDS drop out.

Want a tougher challenge to investigate?
Get at the speed right before the MDS will kick out.
Then try opening the 'camper slide' window
if you have one on your pickup's rear window.
I have tried this and am not sure whether it helps or hurts,
but I suspect it helps slightly.