I'll agree that generally an engine is most efficient at its torque peak. However, with modern engine management systems, the torque curve is relatively flat so it would seem that fuel economy would drop exponentially once you're over about 50 with wind resistance being the primary parasite.
I have had cars in the past that did better at 65 than at 55 but everything I've owned lately (and there have been quite a few) suffers over 65.
And just for reference, the numerous Buick/Pontiac/Olds iterations I've had with the 3800 turn about 2000 rpm at 70 and the two GM 5.3s I have now (Avalanche and Yukon XL) turn about the same.

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1983 M-B 240D-Gone too.
1976 M-B 300D-Departed.

"Good" is the worst enemy of "Great".