is that with the 3.07 rear gear? mine has the curse of the 3.47ish rear gear and going 60mph on long highway runs gives me about 25-27mpg. And yes slow is the way to go!!
CURSE the exponential increase in wind resistance with regards to speed.

F= (1/2)*ρ*v^2*A*C
F=force of wind resistance against the forward momentum
ρ=density of fluid (air is a fluid)
v=velocity
A=Frontal area
C=Drag coefficient

so yes slow is the way to go

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'81 300D - swapped to a 617.951 turbo motor with a 150 amp alt.