using aero rolling resistance calculator
https://ecomodder.com/forum/tool-aer...resistance.php
used these variables I just guessed at_
Vehicle weight: 2721.6 kg / 6000 lbs
Crr: .011
Cd: .45
A: 3 m2 / 32 ft2
Fuel energy density (Wh/US gal.): 33557
Engine efficiency: .22
Drivetrain efficiency: .95
Parasitic overhead (Watts): 0
rho: 1.225 kg/m3
Dropping back from 65mph to 60 mph you mpg would go from 15.92 to 17.77
a gain of 1.85 mpg
Using the other calculator, on 30 mile long trip, you would give up 2 minutes and 18 seconds of time by driving slower.
So you have to decide is 1.85 mpg worth 2 minutes and 18 seconds of your time. It may not be, for others it is.
At $2.60 a gallon, the 30 mile trip would cost you about $7.10, at 65 mph and $6.00 at 60 mph. That roughly equal to 30 bucks an hour. Your time is more than likely worth more than that if you are working for yourself.
On my morning comute, I have 8.4 miles of highway. Dropping back to 60 from 65 only cost me 28 seconds, I decided it was worth it to get better MPG, even though this is just a hobby for me.