I see the confusion now. The 19.5" wheels are close to 20" but the available tire to go on them are a LOT shorter than stock tire size thus the "undernourished" look, and a slower truck.
5" is a lot of difference in tire diameter.

•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-