I think you will want the 0.73.
For the best combination of fuel economy and performance you want the rpm while at your preferred speed, say 72mph to be at the peak torque rpm for the engine. When you hit a hill that you can't pull in OD you downshift to DD.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-