Remember I said the measurement standards changed in the late '40s as best I can make out. So, on your truck they list either the 20 x 3.75 or 20 x 4.33. The 3.75" is the measurement inside along the wheel base between the lips. It was later called a 5" because that's the measurement across from outer rim lip to outer rim lip. Similarly the 4.33" was later called a 6". I just went out to the barn and put a tape on a couple of mine (unmounted) that I know the Budd numbers for and verified the measurements against the book.

The recommended tire for a 3.75" is a 6.00. A 6.50 appears to be acceptable. The recommended for the 4.33" was a 6.50 with a 7.00 acceptable. A 20 x 5.00 rim (7" by the later standard) would be 7.00 recommended with a 7.50 acceptable. Then you get into the 20 x 6.00 rims (8") and they list recommended as a 7.50 tire with 8.25 as acceptable. Obviously you might find tires mounted on rims that they aren't recommended for. That happens. Hope that helps and makes sense. Stu