I would assume you know that a two speed gives you a slow and slower rear ratio.
Some people think you can go faster if you have a two speed rear, not so. If you swap a two speed where you had a single you are most likely swapping to a lower, slower on the road, ratio.

•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-