Actually, I think the purpose of the 2-speed rear was to get more low end pulling power using the lower ratio. The high ratio was about the same that came in the single-speed rears for the Chevy 5-6000 series trucks. I'm assuming GMC is similar.
Curt
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1953 Chevy 6400, 1957 Chevy 2dr Sedan
--"Mediocrity is easy, the good things take time"
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