Here are the two wheels side by side, 18" and 19.5"
PICTURE

This PICTURE
Shows a ruler lined up with outside of wheel mounting surface indicating depth of the 18" wheel center.

Same idea on the 6x19.5" wheel showing the drop center starts about 3.5" from the WMS.
PICTURE

Recall that the OD of the 18" wheel center is about 1/4" larger than the ID of the 19.5" drop center. Therefore they will intersect at the corner of the drop center.

•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-