It does not really make sense does it?
Perhaps the resistance of having 4 more tires on the ground is much worse than we think it is?
I could cary the weight of two empty tralers in the bed of the truck, and still use 4th gear. But I can't drag one and use 4th?

•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-